docs: drop period in headers

We don't do this in most places, so standardize the outliers.

Change-Id: I80c1ef16c5fa7355dcb7797bc8e56852b22c24d5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/583641
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

CONTRIBUTING.md

@@ -24,7 +24,7 @@ However there are some differences, so please review and familiarize
 yourself with the following relevant bits.
 
 
-## Make separate commits for logically separate changes.
+## Make separate commits for logically separate changes
 
 Unless your patch is really trivial, you should not be sending out a patch that
 was generated between your working tree and your commit head.
@@ -118,7 +118,7 @@ your patch.  It is virtually impossible to remove a patch once it
 has been applied and pushed out.
 
 
-## Sending your patches.
+## Sending your patches
 
 Do not email your patches to anyone.
 

docs/windows.md

@@ -145,7 +145,7 @@ You will have to specify this flag every time you upload.
 
 [mintty]: https://github.com/mintty/mintty/issues/56
 
-### repohooks always fail with an close_fds error.
+### repohooks always fail with an close_fds error
 
 When using the [reference repohooks project][repohooks] included in AOSP,
 you might see errors like this when running `repo upload`: