Fix incorrect hint messages about force stop

[yalinhuang]

The hint message is incorrect --

There are two ways you can fix this:

• temporarily: pgctl stop --force playground/sweet
• permanently: http://pgctl.readthedocs.org/en/latest/user/quickstart.html#writing-playground-services

"--force" is an optional argument while "stop" and "playground/sweet" are positional arguments; optional arguments cannot appear in between positional ones. The pgctl command should instead be pgctl --force stop playground/sweet or pgctl stop playground/sweet --force.