Docs: Fix the intersphinx URL of the AiiDA documentation (#910)

The URL `http://aiida_core.readthedocs.io/en/latest/` was returning Bad
Request errors as it seems to have been moved to the URL
`http://aiida.readthedocs.io/en/latest/` instead.

docs/source/conf.py

@@ -60,7 +60,7 @@
 # Setting the intersphinx mapping to other readthedocs
 intersphinx_mapping = {
  'python': ('https://docs.python.org/3.8', None),
- 'aiida': ('http://aiida_core.readthedocs.io/en/latest/', None),
+ 'aiida': ('http://aiida.readthedocs.io/en/latest/', None),
  'aiida_pseudo': ('http://aiida-pseudo.readthedocs.io/en/latest/', None),
 }