[pigeon] Fix typo in README.md (#4707)

Fix a typo in the Pigeon README.md

Author: mit-mit

packages/pigeon/README.md

@@ -21,7 +21,7 @@ Currently pigeon supports generating:
 ### Supported Datatypes
 
 Pigeon uses the `StandardMessageCodec` so it supports 
-[[any datatype platform channels support](https://flutter.dev/docs/development/platform-integration/platform-channels#codec)].
+[any datatype platform channels support](https://flutter.dev/docs/development/platform-integration/platform-channels#codec).
 
 Custom classes and nested datatypes are also supported.