Make help message clearer

I struggled a bit when first using ruby-resty, trying to figure out how to get the help message to show up. The original message is a little bit unclear.

I've just changed the wording so that it will show:

```ruby
resty> GET
Missing path. Type 'GET -h' for more info, or 'help'.
```

But definitely open to other suggestions if you can think of something better.