docs: fix typo "succesfullly" in checkout.dart (flutter-stripe#1013)

Co-authored-by: Jaime Blasco <[email protected]>
Co-authored-by: jonasbark <[email protected]>

Author: 3 people

packages/stripe_checkout/lib/src/platforms/checkout.dart

@@ -9,7 +9,7 @@ class CheckoutResponse with _$CheckoutResponse {
   /// Web only
   const factory CheckoutResponse.redirected() = _Redirect;
 
-  /// The checkout payment has been completed succesfullly
+  /// The checkout payment has been completed succesfully
   /// Mobile only
   const factory CheckoutResponse.success() = _Success;