doc: update precommit.md

This change removes internal information about Google workstations from documentation.

Change-Id: I9284ee56d6fe2364b36e4146a16bfca50e7fc90a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/242779
Reviewed-by: Julie Qiu <julie@golang.org>

doc/precommit.md

@@ -8,8 +8,7 @@ it with
 ln -s -f ../../all.bash .git/hooks/pre-commit
 ```
 
-Google workstations start with a small shell script as a pre-commit hook. To
-add `all.bash` to it, edit `.git/hooks/pre-commit` and add the line
+To add `all.bash` to an existing pre-commit hook, edit `.git/hooks/pre-commit` and add the line
 
 ```
 ./all.bash