Fixed comment referencing old single_process flag

Bug: 460513
Change-Id: Id4a48b7a270305953ca38e1e626f644306a861d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2117324Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753232}

content/public/test/test_launcher.cc

@@ -99,7 +99,7 @@ void PrintUsage() {
           "  --test-launcher-jobs=N\n"
           "    Sets the number of parallel test jobs to N.\n"
           "\n"
-          "  --single_process\n"
+          "  --single-process-tests\n"
           "    Runs the tests and the launcher in the same process. Useful\n"
           "    for debugging a specific test in a debugger.\n"
           "\n"