Remove comment about FieldMask not being supported (#314)
It is not true.
diff --git a/protobuf/lib/src/protobuf/generated_message.dart b/protobuf/lib/src/protobuf/generated_message.dart
index 03ec80c..0944569 100644
--- a/protobuf/lib/src/protobuf/generated_message.dart
+++ b/protobuf/lib/src/protobuf/generated_message.dart
@@ -223,7 +223,6 @@
/// Merges field values from [json], a JSON object using proto3 encoding.
///
/// Well-known types and their special JSON encoding are supported.
- /// Except `FieldMask`.
///
/// If [ignoreUnknownFields] is `false` (the default) an
/// [FormatException] is be thrown if an unknown field name
