Update Action Cable README.md - typo fix

Author: HarryCollins

actioncable/README.md

@@ -53,7 +53,7 @@ module ApplicationCable
 
     private
       def find_verified_user
-        if current_user = User.find_by(id: cookies.signed[:user_id])
+        if current_user == User.find_by(id: cookies.signed[:user_id])
           current_user
         else
           reject_unauthorized_connection