docs: conf.py: update deprecated source_suffix configuration

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

docs/conf.py

@@ -48,7 +48,7 @@ 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 master toctree document.
 master_doc = 'index'