Fix plugins command help output
Fix plugins command help output

This PR removes the redundant `heroku` word here (in "Additional commands" section):
```
→ heroku help plugins
Usage: heroku plugins

 list installed plugins

Example:

 $ heroku plugins
 === Installed Plugins
 heroku-production-check@0.2.0

Additional commands, type "heroku help COMMAND" for more details:

  heroku plugins:link [PATH]  #  This is useful when developing plugins locally.
  plugins:install URL         #  install a plugin
  plugins:uninstall PLUGIN    #  uninstall a plugin
  plugins:update [PLUGIN]     #  updates all plugins or a single plugin by name
```

And also, here:
```
→ heroku help plugins:link
Usage: heroku heroku plugins:link [PATH]
 Links a local plugin into CLI.
 This is useful when developing plugins locally.
 It simply symlinks the specified path into ~/.heroku/node_modules
Example:
 $ heroku plugins:link .
```
dickeyxxx/heroku,rkh/heroku,Faisalawanisee/heroku,dickeyxxx/heroku,gregburek/heroku,dickeyxxx/heroku,vcgato29/heroku,rkh/heroku,vcgato29/heroku,greeca0204/heroku,duypq94/heroku,gregburek/heroku,duypq94/heroku,Faisalawanisee/heroku,vcgato29/heroku,macedo/heroku,sancopanco/heroku,macedo/heroku,Faisalawanisee/heroku,greeca0204/heroku,sancopanco/heroku,rkh/heroku,macedo/heroku,duypq94/heroku,gregburek/heroku,sancopanco/heroku,greeca0204/heroku