conf: exclude swh. part when building the module index

so we have more than just the 's' index entry 's' in there.

swh/docs/sphinx/conf.py

@@ -125,6 +125,8 @@ autodoc_default_flags = ['members', 'undoc-members']
 autodoc_member_order = 'bysource'
 autodoc_mock_imports = ['rados']
 
+modindex_common_prefix = ['swh.']
+
 # for the extlinks extension, sub-projects should fill that dict
 extlinks = {}