add alias descriptions

plugins/gem/README.md

@@ -7,11 +7,11 @@ To use it, add `gem` to the plugins array in your zshrc file:
 ```zsh
 plugins=(... gem)
 ```
-## Aliases
 
-| Alias | Command               |
-|-------|-----------------------|
-| gemb  | `gem build *.gemspec` |
-| gemp  | `gem push *.gem`      |
-| gemy  | `gem yank $1 -v $2`   |
+## Aliases
 
+| Alias                | Command                       | Description                                |
+|----------------------|-------------------------------|--------------------------------------------|
+| gemb                 | `gem build *.gemspec`         | Build a gem from a gemspec                 |
+| gemp                 | `gem push *.gem`              | Push a gem up to the gem server            |
+| gemy [gem] [version] | `gem yank [gem] -v [version]` | Remove a pushed gem version from the index |