Minor update to Sphinx config

The map form of source_suffix was introduced in Sphinx v1.8.0.

docs/conf.py

@@ -43,7 +43,7 @@ templates_path = ['_templates']
 # You can specify multiple suffix as a list of string:
 #
 # source_suffix = ['.rst', '.md']
-source_suffix = '.rst'
+source_suffix = {'.rst': 'restructuredtext'}
 
 # The encoding of source files.
 #