Add backticks around codey-things

Co-authored-by: Duncan Stuart <duncan@varvet.com>

README.md

@@ -199,7 +199,7 @@ you can retrieve it by passing a symbol.
 class DashboardPolicy
   attr_reader :user
 
-  # _record in this example will be :dashboard
+  # `_record` in this example will be :dashboard
   def initialize(user, _record)
     @user = user
   end