pre-push.sample: Write error message to stderr

githooks(5) suggests:

  Information about why the push is rejected may be sent to the user
  by writing to standard error.

So follow that advice in the sample.
Signed-off-by: NW. Trevor King <wking@tremily.us>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

templates/hooks--pre-push.sample

@@ -45,7 +45,7 @@ do
 		commit=`git rev-list -n 1 --grep '^WIP' "$range"`
 		if [ -n "$commit" ]
 		then
-			echo "Found WIP commit in $local_ref, not pushing"
+			echo >&2 "Found WIP commit in $local_ref, not pushing"
 			exit 1
 		fi
 	fi