contrib/credential: 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>

contrib/credential/gnome-keyring/git-credential-gnome-keyring.c

@@ -401,7 +401,7 @@ static void usage(const char *name)
 	const char *basename = strrchr(name,'/');
 
 	basename = (basename) ? basename + 1 : name;
-	fprintf(stderr, "Usage: %s <", basename);
+	fprintf(stderr, "usage: %s <", basename);
 	while(try_op->name) {
 		fprintf(stderr,"%s",(try_op++)->name);
 		if(try_op->name)

contrib/credential/osxkeychain/git-credential-osxkeychain.c

@@ -154,7 +154,7 @@ static void read_credential(void)
 int main(int argc, const char **argv)
 {
 	const char *usage =
-		"Usage: git credential-osxkeychain <get|store|erase>";
+		"usage: git credential-osxkeychain <get|store|erase>";
 
 	if (!argv[1])
 		die(usage);

contrib/credential/wincred/git-credential-wincred.c

@@ -313,7 +313,7 @@ static void read_credential(void)
 int main(int argc, char *argv[])
 {
 	const char *usage =
-	    "Usage: git credential-wincred <get|store|erase>\n";
+	    "usage: git credential-wincred <get|store|erase>\n";
 
 	if (!argv[1])
 		die(usage);