Fix usage message for missing mountpoint argument.

Change-Id: I5e9a1822c9239d6da47569af91158f2f821b47ea

cmd/slothfs-multifs/main.go

diff --git a/cmd/slothfs-multifs/main.go b/cmd/slothfs-multifs/main.go
index c8b8e24..2a88277 100644
--- a/cmd/slothfs-multifs/main.go
+++ b/cmd/slothfs-multifs/main.go
@@ -43,8 +43,9 @@
 	}
 
 	if len(flag.Args()) < 1 {
-		log.Fatal("usage: main -gitiles URL -repo REPO [-branch BRANCH] MOUNT-POINT")
+		log.Fatal("mountpoint argument missing.")
 	}
+
 	mntDir := flag.Arg(0)
 
 	cache, err := cache.NewCache(*cacheDir, cache.Options{})