[docs] Update IntelliJ plugin link
[docs] Update IntelliJ plugin link

The previously-linked project hasn't been updated for years.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf