[google_sign_in] Fix Obj-C formatting bug (flutter#5310)

Fix a formatting mistake from the conversion to Pigeon; the `;` here doesn't affect the behavior, but it's confusing, and makes the autoformatter unhappy.

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_ios/ios/Classes/FLTGoogleSignInPlugin.m

@@ -173,8 +173,9 @@ - (void)getAccessTokenWithCompletion:(nonnull void (^)(FSITokenData *_Nullable,
   }];
 }
 
-- (void)signOutWithError:(FlutterError *_Nullable *_Nonnull)error;
-{ [self.signIn signOut]; }
+- (void)signOutWithError:(FlutterError *_Nullable *_Nonnull)error {
+  [self.signIn signOut];
+}
 
 - (void)disconnectWithCompletion:(nonnull void (^)(FlutterError *_Nullable))completion {
   [self.signIn disconnectWithCallback:^(NSError *error) {