Update conf.py to be compatible with Sphinx 4.1.2

Change-Id: I905911121fca48ae85f046bef714135cbf641d0a

docs/source/conf.py

diff --git a/docs/source/conf.py b/docs/source/conf.py
index c83468f..cc55b8a 100644
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -246,7 +246,7 @@
 # By default MathJax does not use TeX fonts, which is a tragedy. Also
 # scaling the fonts down a bit makes them fit better with font sizing
 # in the "Read The Docs" theme.
-mathjax_config = {
+mathjax3_config = {
 'HTML-CSS' : {
         'availableFonts' : ["TeX"],
         'scale' : 90