Correct authentication proc example

Author: bf4

README.md

@@ -45,7 +45,7 @@ Set admin username and password in an initializer:
 
 ApiDocServer.configure do |config|
   config.authentication_proc = proc do |controller|
-    authenticate_or_request_with_http_basic do |username, password|
+    controller.authenticate_or_request_with_http_basic do |username, password|
       User.find_by(name: username, password: password)
     end
   end