Change to absolute path for contrib/docker

because this file is reproduced on dockerhub and relative paths don't work

docker/README.md

@@ -14,7 +14,7 @@ This image is designed to run either with an automatically generated
 configuration file or with a custom configuration that requires manual editing.
 
 An easy way to make use of this image is via docker-compose. See the
-[contrib/docker](../contrib/docker) section of the synapse project for
+[contrib/docker](https://github.com/matrix-org/synapse/tree/master/contrib/docker) section of the synapse project for
 examples.
 
 ### Without Compose (harder)