docs: handle deprecation warning wrt. emojis (#116)

### Summary of Changes

Update Material for Mkdocs configuration to handle a deprecation
warning.

mkdocs.yml

@@ -106,8 +106,8 @@ markdown_extensions:
 
   # Icons & emojis
   - pymdownx.emoji:
-      emoji_index: !!python/name:materialx.emoji.twemoji
-      emoji_generator: !!python/name:materialx.emoji.to_svg
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
 
   # Images
   - attr_list