docs: Clarify commit message footers

Both "Fixed" and "Fixes" are supported (for Buganizer; Monorail only
supports Fixed, see b/206842834). This section used to be somewhat
confusing, because it mentioned only "Fixed" in the body, but used
"Fixes" in the example.

Change-Id: I53f4f97d6cb748951b0592bd4995297538ba35dd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/112317
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Chad Norvell <chadnorvell@google.com>
diff --git a/docs/style_guide.rst b/docs/style_guide.rst
index 7a7c066..69b4a2f 100644
--- a/docs/style_guide.rst
+++ b/docs/style_guide.rst
@@ -1277,7 +1277,8 @@
       Bug: 123
       Bug: 456
 
-* ``Fixed``: Like ``Bug``, but automatically closes the bug when submitted.
+* ``Fixed`` or ``Fixes``: Like ``Bug``, but automatically closes the bug when
+  submitted.
 
   .. code:: none