Fix sphinx warning

Author: jpadilla

docs/conf.py

@@ -153,7 +153,7 @@ def get_version(package):
 html_static_path = ['_static']
 
 html_context = {
-    'css_files': [
+    'extra_css_files': [
         # override wide tables in RTD theme
         '_static/theme_overrides.css',
     ],

docs/installation.rst

@@ -8,7 +8,7 @@ You can install ``PyJWT`` with ``pip``:
     $ pip install pyjwt
 
 Cryptographic Dependencies (Optional)
--------------------
+-------------------------------------
 
 If you are planning on encoding or decoding tokens using certain digital
 signature algorithms (like RSA or ECDSA), you will need to install the