mkdocs: update permalink icon

Author: howardlau1999

mkdocs.yml

@@ -83,7 +83,7 @@ markdown_extensions:
   - def_list
   - markdown.extensions.toc:
       slugify: !!python/object/apply:pymdownx.slugs.slugify {kwds: {case: lower}}
-      permalink: "\ue157"
+      permalink: true
   - pymdownx.emoji:
       emoji_index: !!python/name:materialx.emoji.twemoji
       emoji_generator: !!python/name:materialx.emoji.to_svg