Update find_by_user_id to use an integer

Author: erskingardner

README.md

@@ -42,7 +42,7 @@ The API supports:
 user = Intercom::User.find_by_email("[email protected]")
 user.custom_data["average_monthly_spend"] = 1234.56
 user.save
-user = Intercom::User.find_by_user_id("[email protected]")
+user = Intercom::User.find_by_user_id("1")
 user = Intercom::User.create(:email => "[email protected]", :name => "Bob Smith")
 user = Intercom::User.new(params)
 user.save