Update link to Pebble layer specification

As of a few months ago we have a proper docs site and the README is minimal, so update this link.

docs/explanation/pebble.rst

@@ -84,4 +84,4 @@ convert a Docker entrypoint to a Pebble layer.
 .. _tini: https://github.com/krallin/tini
 .. _s6-overlay: https://github.com/just-containers/s6-overlay
 .. _imperative wrapper scripts (as suggested in the Docker documentation): https://docs.docker.com/engine/containers/multi-service_container/#use-a-wrapper-script
-.. _Pebble layer specification: https://github.com/canonical/pebble#layer-specification
+.. _Pebble layer specification: https://canonical-pebble.readthedocs-hosted.com/en/latest/reference/layer-specification/