Remove from usage msg the -m flag that is not implemented or documented. Probably a typo.

Reference: /n/sources/patch/applied/createuser-unused-usage
Date: Sat Jun 11 15:39:12 CES 2005

--- /sys/src/cmd/auth/changeuser.c	Sat Jun 11 15:37:42 2005
+++ /sys/src/cmd/auth/changeuser.c	Sat Jun 11 15:37:39 2005
@@ -10,7 +10,7 @@
 void
 usage(void)
 {
-	fprint(2, "usage: changeuser [-pnm] user\n");
+	fprint(2, "usage: changeuser [-pn] user\n");
 	exits("usage");
 }