pscp: Fix short help message.

Typing just 'pscp' for help doesn't work since ecfa6b2734 (and is a
particularly infuriating suggestion if that's what you did to elicit
this message).

pscp.c

@@ -2253,7 +2253,7 @@ void cmdline_error(const char *p, ...)
     va_start(ap, p);
     vfprintf(stderr, p, ap);
     va_end(ap);
-    fprintf(stderr, "\n      try typing just \"pscp\" for help\n");
+    fprintf(stderr, "\n      try typing \"pscp -h\" for help\n");
     exit(1);
 }