doc/contribute.html: remove superfluous article

After CL 113016 the "a" article is no longer necessary.

Change-Id: I5a80a210bd2b9eedd73d5f9f3f338d7f22c29ea6
Reviewed-on: https://go-review.googlesource.com/113355Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -838,7 +838,7 @@ The Gerrit voting system involves an integer in the range -2 to +2:
 <p>
 After the code has been +2'ed, an approver will
 apply it to the master branch using the Gerrit user interface.
-This is called a "submitting the change".
+This is called "submitting the change".
 </p>
 
 <p>