Update README code example for writing Custom linters with correct sy…

…ntax. The Custom linter code can be copy/pasted and will be working out of the box now (#335)

README.md

@@ -539,16 +539,13 @@ module ERBLint
       end
       self.config_schema = ConfigSchema
 
-      def offenses(processed_source)
-        errors = []
+      def run(processed_source)
         unless processed_source.file_content.include?('this file is fine')
-          errors << Offense.new(
-            self,
+          add_offense(
             processed_source.to_source_range(0 ... processed_source.file_content.size),
             "This file isn't fine. #{@config.custom_message}"
           )
         end
-        errors
       end
     end
   end