docs: HTTPS link to remark.js.org (#1251)

Author: coliff

docs/THEMING.md

@@ -37,6 +37,6 @@ for some ideas.
 
 The default Markdown generator for documentation.js isn't customizable - instead
 of a plain-text theme, it's generated by creating an AST and then rendering
-it with [remark](http://remark.js.org/). If you need something extra in Markdown,
+it with [remark](https://remark.js.org/). If you need something extra in Markdown,
 you can either rally for that thing to be included in the default theme,
 or you can hack around it by using an HTML theme that outputs Markdown.