docs: fix multiline xvfb-run command

[TechnologyClassroom]

Summary

This is a documentation fix as the xvfb-run command for headless chromium does not currently work.

This PR adds a trailing backslash to a multiline command. Without the trailing backslash, the command is incomplete when pasted.

Output without the change looks something like this (There are other warnings that can be ignored):

/etc/chromium.d/README: line 1: Any: command not found                                                                                      
dpkg-query: no packages found matching bluealsa
[17369:17369:1027/092051.805408:ERROR:component_loader.cc(193)] Failed to parse extension manifest.
[17435:17435:1027/092052.296294:ERROR:gpu_init.cc(486)] Passthrough is not supported, GL is egl, ANGLE is
[17435:17435:1027/092052.369356:ERROR:viz_main_impl.cc(186)] Exiting GPU process due to errors during initialization
[17521:17521:1027/092052.734923:ERROR:gpu_init.cc(486)] Passthrough is not supported, GL is egl, ANGLE is
[17521:17521:1027/092052.807645:ERROR:viz_main_impl.cc(186)] Exiting GPU process due to errors during initialization
[17547:17547:1027/092052.935525:ERROR:gpu_memory_buffer_support_x11.cc(44)] dri3 extension not supported.
[17513:7:1027/092052.949790:ERROR:command_buffer_proxy_impl.cc(128)] ContextResult::kTransientFailure: Failed to send GpuControl.CreateComma
ndBuffer.
^C[17547:17547:1027/092531.004289:ERROR:connection.cc(46)] X connection error received.
[17547:17558:1027/092531.013258:ERROR:connection.cc(46)] X connection error received.
-bash: --start-maximized: command not found

lighthouse then looks like this when it tries to connect (There are other warnings that can be ignored):

  LH:ChromeLauncher No debugging port found on port 9222, launching a new Chrome. +0ms
  LH:ChromeLauncher Waiting for browser. +88ms
  LH:ChromeLauncher Waiting for browser... +1ms
...
  LH:ChromeLauncher Waiting for browser.....................................................................................................
.. +503ms
  LH:ChromeLauncher:error connect ECONNREFUSED 127.0.0.1:9222 +4ms
  LH:ChromeLauncher:error Logging contents of /tmp/lighthouse.OycuoNt/chrome-err.log +0ms
  LH:ChromeLauncher:error /etc/chromium.d/README: line 1: Any: command not found
  LH:ChromeLauncher:error dpkg-query: no packages found matching bluealsa
  LH:ChromeLauncher:error [17757:17757:1027/092133.850895:ERROR:ozone_platform_x11.cc(247)] Missing X server or $DISPLAY
  LH:ChromeLauncher:error [17757:17757:1027/092133.851052:ERROR:env.cc(226)] The platform failed to initialize.  Exiting.
  LH:ChromeLauncher:error  +1ms
Unable to connect to Chrome

With the change the xvfb-run command succeeds (There are other warnings that can be ignored):

/etc/chromium.d/README: line 1: Any: command not found
dpkg-query: no packages found matching bluealsa
[18975:18975:1027/092559.171562:ERROR:component_loader.cc(193)] Failed to parse extension manifest.

DevTools listening on ws://127.0.0.1:9222/devtools/browser/08b41b3e-1299-4582-9d54-bea461031b55
[19041:19041:1027/092559.335451:ERROR:gpu_memory_buffer_support_x11.cc(44)] dri3 extension not supported.
[18975:18975:1027/092611.879392:ERROR:devtools_http_handler.cc(636)] Using unsafe HTTP verb GET to invoke /json/new. This action will stop supporting GET and POST verbs in future versions.

lighthouse is then able to connect and generate reports.

Tested on an arm processor without X11 using node v18.12.0 (npm v8.19.2).

Related Issues/PRs

[google-cla]

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[TechnologyClassroom]

I signed the CLA, rescan did not change the status. PR change is a minor two character change which is not copyrightable.