Update fastlane iOS beta tester link (#2785)

fastlane/Fastfile

@@ -466,7 +466,7 @@ platform :android do
     version_number = android_get_version_name(
         gradle_file: './android/app/build.gradle'
     )
-    
+
    send_message_to_mattermost({
                                   :version_number => version_number,
                                   :build_number => build_number,
@@ -607,7 +607,7 @@ def submit_to_store
 
     if ENV['BETA_BUILD'] == 'true'
       pretext = '#### New iOS beta published to TestFlight'
-      msg = 'Sign up as a beta tester [here](https://mattermost-fastlane.herokuapp.com/)'
+      msg = 'Sign up as a beta tester [here](https://testflight.apple.com/join/Q7Rx7K9P)'
     end
 
     version = get_version_number(xcodeproj: './ios/Mattermost.xcodeproj', target: 'Mattermost')