Fix copy-paste-o in MethodChannel.invokeListMethod doc (flutter#149976)

This sentence didn't make a lot of sense as written.  It looks like
it was probably based on the doc of invokeMapMethod, and then
incompletely adapted.  So, fix it.

Author: gnprice

packages/flutter/lib/src/services/platform_channel.dart

@@ -511,7 +511,7 @@ class MethodChannel {
   ///
   /// Dart generics are reified, meaning that an untyped `List<dynamic>` cannot
   /// masquerade as a `List<T>`. Since [invokeMethod] can only return dynamic
-  /// maps, we instead create a new typed list using [List.cast].
+  /// lists, we instead create a new typed list using [List.cast].
   ///
   /// See also:
   ///