use appropriate type for `rc` option

This fixes the following warning.

```
Expected boolean default value for '--rc'; got "~/.railsrc" (string)
```

railties/lib/rails/commands/plugin/plugin_command.rb

@@ -11,7 +11,7 @@ def self.banner(*) # :nodoc:
         "#{executable} new [options]"
       end
 
-      class_option :rc, type: :boolean, default: File.join("~", ".railsrc"),
+      class_option :rc, type: :string, default: File.join("~", ".railsrc"),
         desc: "Initialize the plugin command with previous defaults. Uses .railsrc in your home directory by default."
 
       class_option :no_rc, desc: "Skip evaluating .railsrc."