doc: fix esplora's docker image name in README

[harshjv]

Esplora's docker image is available at blockstream/esplora

[shesek]

These docker run instructions refer to the docker build instruction above them, which would build this as an image named esplora.

Perhaps you could instead add a sentence under the "How to build the Docker image" section, saying something like "You can also use the pre-build image at blockstream/esplora instead of building it locally"?.