FIX: Incorrect plugin name in `requires_plugin` (#133)

This resulted in `Required plugin 'docs' not found` warnings in logs

app/controllers/docs/docs_controller.rb

@@ -2,7 +2,7 @@
 
 module Docs
   class DocsController < ApplicationController
-    requires_plugin "docs"
+    requires_plugin PLUGIN_NAME
 
     skip_before_action :check_xhr, only: [:index]
 

plugin.rb

@@ -23,6 +23,10 @@ load File.expand_path("lib/docs/query.rb", __dir__)
 
 GlobalSetting.add_default :docs_path, "docs"
 
+module ::Docs
+  PLUGIN_NAME = "discourse-docs"
+end
+
 after_initialize do
   require_dependency "search"