doc: fix link to the reflection protocol (#6833)

Author: y-yagi

reflection/README.md

@@ -2,7 +2,7 @@
 
 Package reflection implements server reflection service.
 
-The service implemented is defined in: https://github.com/grpc/grpc/blob/master/src/proto/grpc/reflection/v1alpha/reflection.proto.
+The service implemented is defined in: https://github.com/grpc/grpc/blob/master/src/proto/grpc/reflection/v1/reflection.proto.
 
 To register server reflection on a gRPC server:
 ```go