Documentation/user-manual.txt: use a lowercase "usage:" string

Make the usage string in the example script 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>

Documentation/user-manual.txt

@@ -2341,7 +2341,7 @@ origin)
 	fi
 	;;
 *)
-	echo "Usage: $0 origin|test|release" 1>&2
+	echo "usage: $0 origin|test|release" 1>&2
 	exit 1
 	;;
 esac
@@ -2355,7 +2355,7 @@ pname=$0
 
 usage()
 {
-	echo "Usage: $pname branch test|release" 1>&2
+	echo "usage: $pname branch test|release" 1>&2
 	exit 1
 }