Fix typo in GeneratedMessage.copyWith documentation (#641)

protobuf/lib/src/protobuf/generated_message.dart

diff --git a/protobuf/lib/src/protobuf/generated_message.dart b/protobuf/lib/src/protobuf/generated_message.dart
index 40198da..c9cb0a9 100644
--- a/protobuf/lib/src/protobuf/generated_message.dart
+++ b/protobuf/lib/src/protobuf/generated_message.dart
@@ -82,7 +82,7 @@
 
   /// Apply [updates] to a copy of this message.
   ///
-  /// Makes a writable shawwol copy of this message, applies the [updates] to
+  /// Makes a writable shallow copy of this message, applies the [updates] to
   /// it, and marks the copy read-only before returning it.
   @Deprecated('Using this can add significant size overhead to your binary. '
       'Use [GeneratedMessageGenericExtensions.rebuild] instead. '