Fixed docs build error

https://sphinxcontrib-napoleon.readthedocs.io/en/latest/
"As of Sphinx 1.3, the napoleon extension will come packaged with Sphinx under sphinx.ext.napoleon. The sphinxcontrib.napoleon extension will continue to work with Sphinx <= 1.2."

doc/source/conf.py

@@ -33,7 +33,7 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['sphinx.ext.autodoc', 'sphinxcontrib.napoleon']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.napoleon']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']