types/dynamicpb: minor doc change
types/dynamicpb: minor doc change

Change-Id: I13439b810e18142d85f4466be006d3abf3a66c30
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219502
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go