settings.json: in the mysql example, mention the existence of port parameter

The port parameter is supported by the mysql driver, but to know this one has to
read through ueberDB's code.

At least for this simple case, it may make sense to be explicit.

Fixes #3453

settings.json.template

@@ -83,6 +83,7 @@
   "dbSettings" : {
                    "user"    : "etherpaduser",
                    "host"    : "localhost",
+                   "port"    : 3306,
                    "password": "PASSWORD",
                    "database": "etherpad_lite_db",
                    "charset" : "utf8mb4"