docs: add missing '.' at start of Sphinx link

This worked fine on the actual page, but in the summary table on the
parent kazu/utils page, it couldn't find where the class came from, so
gave an error.

kazu/utils/spacy_object_mapper.py

@@ -1,4 +1,4 @@
-"""Provides :class:`KazuToSpacyObjectMapper`.
+"""Provides :class:`.KazuToSpacyObjectMapper`.
 
 This class used to be called ``SpacyToKazuObjectMapper`` but this naming was confusing.
 The old name will still work for now, but using it produces a