update intersphinx mapping (#10074)

this hopefully fixes our nightly failures

readthedocs seems to redirect users to its .io site so
https://acme-python.readthedocs.org/en/latest/objects.inv is supposed to
redirect people to
https://acme-python.readthedocs.io/en/latest/objects.inv, but that
doesn't always seem to work and instead [sometimes serves a
403](https://dev.azure.com/certbot/certbot/_build/results?buildId=8237&view=logs&j=d74e04fe-9740-597d-e9fa-1d0400037dfd&t=dde413a4-f24c-59a0-9684-e33d79f9aa02&l=800)

removing the need for this redirect seems to fix things based on some
quick testing and certainly shouldn't hurt

certbot/docs/conf.py

@@ -314,5 +314,5 @@ texinfo_documents = [
 
 intersphinx_mapping = {
     'python': ('https://docs.python.org/', None),
-    'acme': ('https://acme-python.readthedocs.org/en/latest/', None),
+    'acme': ('https://acme-python.readthedocs.io/en/latest/', None),
 }