using_configuring_artifact_server.rst: Fix incorrect reference to bst push (with link)

When an artifact is built locally, BuildStream will try to push it to all the

caches which have the ``push: true`` flag set. You can also manually push

artifacts to a specific cache using the :ref:`bst pull command <commands>`.

artifacts to a specific cache using the :ref:`bst push command <invoking_push>`.

Artifacts are identified using the element's :ref:`cache key <cachekeys>` so

the builds provided by a cache should be interchangable with those provided