Fix typo on gpu testing docs.

'o' should have been '#' in one section.

Bug: None
Change-Id: I458e1f7300e8ea9a2607d23ee1d80e039acc9879
Tbr: kbr@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879308Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709078}

docs/gpu/gpu_testing.md

@@ -459,7 +459,7 @@ invoke it via:
 
 [new-isolates]: gpu_testing_bot_details.md#Adding-a-new-isolated-test-to-the-bots
 
-o## Adding new steps to the GPU Bots
+### Adding new steps to the GPU Bots
 
 The tests that are run by the GPU bots are described by a couple of JSON files
 in the Chromium workspace: