fix: prevent footnote back arrow from becoming an emoji (cotes2020#1716)

Author: huanyushi

_config.yml

@@ -151,6 +151,7 @@ baseurl: ""
 # ------------ The following options are not recommended to be modified ------------------
 
 kramdown:
+  footnote_backlink: "↩︎"
   syntax_highlighter: rouge
   syntax_highlighter_opts: # Rouge Options › https://github.com/jneen/rouge#full-options
     css_class: highlight