Added example usage for the default port

According to Issue ##7756 I added the example usage.

docs/tutorial/debugging-main-process.md

@@ -16,6 +16,12 @@ Electron will listen for V8 debugger protocol messages on the specified `port`,
 an external debugger will need to connect on this port. The default `port` is
 `5858`.
 
+An example debug command on the default port:
+
+```
+$ node_modules/.bin/electron --debug=5859 .
+# main process runs... 
+```
 ### `--debug-brk=[port]`
 
 Like `--debug` but pauses execution on the first line of JavaScript.