Update protobuf link to flatbuffer

Ref: https://github.com/ry/deno/pull/330

README.md

@@ -22,7 +22,7 @@
  code. Defaults to read-only file system access and no network access.
  Access between V8 (unprivileged) and Golang (privileged) is only done via
  serialized messages defined in this
- [protobuf](https://github.com/ry/deno/blob/master/src/msg.proto). This makes it
+ [flatbuffer](https://github.com/ry/deno/blob/master/src/msg.fbs). This makes it
  easy to audit.
  To enable write access explicitly use `--allow-write` and `--allow-net` for
  network access.