message: doc: Fix image in top level README
diff: </div>
  <p align="center">
  - <img src="https://raw.githubusercontent.com/buttplugio/buttplug-rs/dev/buttplug/docs/buttplug_rust_docs.png">
  + <img src="https://raw.githubusercontent.com/buttplugio/buttplug/master/images/buttplug_rust_docs.png">
  </p>
  A Rust implementation of the Buttplug Intimate Hardware Control Protocol, including a client and server. This is the core implementation of Buttplug.
  