Ask to restart flipper to pick up ios SDK

Summary: Running a check is run it only executes the `run` function. The `run` function for `ios.sdk` is a pure function that detects if ios SDK available from `EnvironmentInfo`. This info is picked up when flipper first started. Even if you installed ios SDK flipper won't pick it up while running. Restarting flipper will pick up the SDK and make it available. We should be clear about it.

Reviewed By: mweststrate

Differential Revision: D51614258

fbshipit-source-id: 8b2a8b5b94ec0038acf9f383e55e166c52b2ccab

desktop/doctor/src/fb-stubs/messages.tsx

@@ -18,7 +18,7 @@ export const installXcode =
   'Install Xcode from the App Store or download it from https://developer.apple.com';
 
 export const installSDK =
-  'You can install it using Xcode (https://developer.apple.com/xcode/).';
+  'You can install it using Xcode (https://developer.apple.com/xcode/). Once installed, restart flipper.';
 
 export const installAndroidStudio = [
   'Android Studio is not installed.',