Update source for grpc/swift-protobuf (#1961)

https://github.com/grpc/grpc-swift-protobuf had the initial v2 plugin
version, which then moved to https://github.com/grpc/grpc-swift-2.

Author: stefanvanburen

plugins/grpc/swift-protobuf/source.yaml

@@ -1,4 +1,6 @@
 source:
   github:
+    # https://github.com/grpc/grpc-swift-protobuf had the initial v2 plugin version,
+    # which then moved to https://github.com/grpc/grpc-swift-2.
     owner: grpc
-    repository: grpc-swift-protobuf
+    repository: grpc-swift-2