doc/contribute.html: fix name of .gitcookies file

Change-Id: I41c22b9c6933b3f3469c0e815048a49e1d37927a
Reviewed-on: https://go-review.googlesource.com/54190Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -88,7 +88,7 @@ script as *nix.)
 </p>
 
 <p>
-Your secret authentication token is now in a <code>.gitcookie</code> file
+Your secret authentication token is now in a <code>.gitcookies</code> file
 and Git is configured to use this file.
 </p>