templates/hooks--update.sample: use a lowercase "usage:" string

Make the usage string consistent with Git.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--update.sample

@@ -38,7 +38,7 @@ if [ -z "$GIT_DIR" ]; then
 fi
 
 if [ -z "$refname" -o -z "$oldrev" -o -z "$newrev" ]; then
-	echo "Usage: $0 <ref> <oldrev> <newrev>" >&2
+	echo "usage: $0 <ref> <oldrev> <newrev>" >&2
 	exit 1
 fi