python.md: Update .style.yapf example to for PEP-8

TBR=agrieve  # trivial doc update.

Bug: 846432
Change-Id: I996bb72278c17c8af6840a16b38808f29727a4f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989765
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728952}

styleguide/python/python.md

@@ -53,7 +53,7 @@ Directories can opt into enforcing auto-formatting by adding a `.style.yapf`
 file with the following contents:
 ```
 [style]
-based_on_style = chromium
+based_on_style = pep8
 ```
 
 Entire files can be formatted (rather than just touched lines) via: