tools/xs-clients: fix usage information of xenstore-exists

The usage information of xenstore-exists is just wrong, as the "-p"
option is not supported by xenstore-exists.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/xs-clients/xenstore_client.c

@@ -232,7 +232,7 @@ usage(int err_val, enum mode mode, int incl_mode, const char *progname)
 	errx(err_val, "Usage: %s %s[-h] [-t] key [...]", progname, mstr);
     case MODE_exists:
 	mstr = incl_mode ? "exists " : "";
-	/* fallthrough */
+	errx(err_val, "Usage: %s %s[-h] key", progname, mstr);
     case MODE_list:
 	mstr = mstr ? : incl_mode ? "list " : "";
 	errx(err_val, "Usage: %s %s[-h] [-p] key [...]", progname, mstr);