Update Pixel Test Docs

Update the example usage of --local-run for pixel tests.

This is a follow-up to a change of how the argument is used:
https://chromium-review.googlesource.com/c/chromium/src/+/1860677

Bug: 1013358
Change-Id: Id1ccdd5b55c89e976ff77853925c0db9c7bcd835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862624
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706024}

docs/gpu/gpu_testing.md

@@ -292,12 +292,14 @@ comparison. When using `--no-skia-gold-failure`, you'll also need to pass the
 `--passthrough` flag in order to actually see the link output.
 
 Example usage:
-`run_gpu_integration_test.py pixel --no-skia-gold-failure --local-run
---passthrough --build-revision aabbccdd`
-
-Note that `aabbccdd` must be replaced with an actual Chromium src revision
-(typically whatever revision origin/master is currently synced to) in order for
-the tests to work. This can be done automatically using:
+`run_gpu_integration_test.py pixel --no-skia-gold-failure --local-run=1
+--passthrough`
+
+If, for some reason, the local run code is unable to determine what the git
+revision is, simply pass '--build-revision aabbccdd'. Note that `aabbccdd` must
+be replaced with an actual Chromium src revision (typically whatever revision
+origin/master is currently synced to) in order for the tests to work. This can
+be done automatically using:
 ``run_gpu_integration_test.py pixel --no-skia-gold-failure --local-run
 --passthrough --build-revision `git rev-parse origin/master` ``