message: fix link to ubuntu dockerfile
diff: RUST_TEST_THREADS=1 TEST_POOL_IP=10.0.0.2 cargo test
  ```
  -See [ci/ubuntu.dockerfile](https://github.com/hyperledger/indy-sdk/tree/master/ci/ubuntu.dockerfile) for example of Ubuntu based environment creation in Docker.
  +See [libindy/ci/ubuntu.dockerfile](https://github.com/hyperledger/indy-sdk/tree/master/libindy/ci/ubuntu.dockerfile) for example of Ubuntu based environment creation in Docker.
  