docs: --force-fieldtrials was h2 rather than h3 (#29161)

All the other argument headers were h3 (`###`) but `--force-fieldtrials` was h2 (`##`) for some reason. 
I changed it to make it consistent with the others.

docs/api/command-line-switches.md

@@ -75,7 +75,7 @@ This switch can not be used in `app.commandLine.appendSwitch` since it is parsed
 earlier than user's app is loaded, but you can set the `ELECTRON_ENABLE_LOGGING`
 environment variable to achieve the same effect.
 
-## --force-fieldtrials=`trials`
+### --force-fieldtrials=`trials`
 
 Field trials to be forcefully enabled or disabled.