Fix typo in plugins doc

The command ``bin/rails plugin`` does not exists.
For me it was showing:

```shell
$ rails plugin --help
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in
`require': cannot load such file -- rails/commands/plugin (LoadError)
 from
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in
`block in require'
 from
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in
`load_dependency'
from
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in
`require'
 from
/usr/local/rvm/gems/ruby-1.9.3-p392/gems/railties-4.0.0/lib/rails/commands.rb:49:in
`<top (required)>'
 from bin/rails:4:in `require'
from bin/rails:4:in `<main>'
​
​```​

​Maybe is fixed in a newer version.​