Update README to reference pythd image tag (#187)

Author: tompntn

README.md

@@ -26,7 +26,7 @@ You can run the following command to open a shell in a linux docker container wi
 ```
 export PYTH_REPO=/path/to/host/pyth-client
 
-// Use the latest mainnet image from https://hub.docker.com/r/pythfoundation/pyth-client/tags?name=mainnet
+// Use the latest pythd image from https://hub.docker.com/r/pythfoundation/pyth-client/tags?name=pythd
 export IMAGE=...
 
 docker run -it \