Fix --maximum_edition usage text (#15828)

For the conformance test runner, `--maximum` is not an option, but `--maximum_edition` is.

Closes #15828

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/15828 from rgov:patch-1 5f123ca5fe
PiperOrigin-RevId: 606820552

conformance/conformance_test_runner.cc

@@ -123,9 +123,9 @@ void UsageError() {
           "                              strictly conforming to protobuf\n");
   fprintf(stderr, "                              spec.\n");
   fprintf(stderr,
-          "  --maximum edition           Only run conformance tests up to \n");
+          "  --maximum_edition <edition>   Only run conformance tests up\n");
   fprintf(stderr,
-          "                              and including the specified\n");
+          "                              to and including the specified\n");
   fprintf(stderr, "                              edition.\n");
   fprintf(stderr,
           "  --output_dir                <dirname> Directory to write\n"