Change sample code to use "microsoft.com" instead of string constant, which will be remove soon. (#2501)

Author: Yue-Wang-Google

Example/Auth/Sample/MainViewController.m

@@ -1850,7 +1850,7 @@ - (void)signInGoogleHeadfulLite {
     @brief Invoked when "Sign in with Microsoft (headful-lite)" row is pressed.
  */
 - (void)signInMicrosoftHeadfulLite {
-  FIROAuthProvider *provider = [FIROAuthProvider providerWithProviderID:FIRMicrosoftAuthProviderID];
+  FIROAuthProvider *provider = [FIROAuthProvider providerWithProviderID:@"microsoft.com"];
   provider.customParameters = @{
     @"prompt" : @"consent",
     @"login_hint" : @"[email protected]",