vtest: fix typo in usage-string

We don't have an --use-egl option, we have a --use-gles option. This was
simply a typo when I updated the string last.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>

vtest/vtest_server.c

@@ -190,7 +190,7 @@ static void vtest_main_parse_args(int argc, char **argv)
          break;
       default:
          printf("Usage: %s [--no-fork] [--no-loop-or-fork] [--use-glx] "
-                "[--use-egl-surfaceless] [--use-egl] [file]\n", argv[0]);
+                "[--use-egl-surfaceless] [--use-gles] [file]\n", argv[0]);
          exit(EXIT_FAILURE);
          break;
       }