Update help info for consistency

The current help output seems to be missing square brackets around the `keys:remove` command example.

```
› heroku help keys
...

Additional commands, type "heroku help COMMAND" for more details:

  keys:add [KEY]      #  add a key for the current user
  keys:clear          #  remove all authentication keys from the current user
  keys:remove KEY     #  remove a key from the current user
  keys:search [TERM]  #  Find keys by fingerprint or contents
```

This changes fixes that by adding the square brackets around the word "KEY".