sphinx: Fix reference to persistent-identifier

This uses `swh-docs/swh/docs/sphinx/conf.py:intersphinx_mapping` config to resolve the
link.

Part of the broken build [1]

[1] https://jenkins.softwareheritage.org/job/DDOC/job/dev/6791/console

Related to T3648

docs/getting-started.rst

@@ -21,7 +21,7 @@ First, ask the Vault to prepare your bundle:
 
     curl -X POST https://archive.softwareheritage.org/api/1/vault/flat/:swhid/
 
-where ``:swhid`` is a :ref:`persistent-identifier`. This initial request and all
+where ``:swhid`` is a :ref:`swh-devel:persistent-identifier`. This initial request and all
 subsequent requests to this endpoint will return some JSON data containing
 information about the progress of bundle creation:
 

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3648 is now swh-docs#3648 (closed)