Fixed: Changed links to mkdocs.org to HTTPS

To prevent the unnecessary HTTP 301 (and reduce the redirects noise
when running the W3C link checker tool against generated pages).

Also fixes the anchor fragment for `#using-the-theme-custom_dir`.

docs/customization.md

@@ -13,7 +13,7 @@ necessary to preserve the desired style.
 few tweaks to an existing theme, you can just add your stylesheets and
 JavaScript files to the `docs` directory.
 
-  [1]: http://www.mkdocs.org
+  [1]: https://www.mkdocs.org
 
 ### Additional stylesheets
 
@@ -57,7 +57,7 @@ extra_javascript:
 
 Further assistance can be found in the [MkDocs documentation][2].
 
-  [2]: http://www.mkdocs.org/user-guide/styling-your-docs/#customizing-a-theme
+  [2]: https://www.mkdocs.org/user-guide/styling-your-docs/#customizing-a-theme
 
 ## Extending the theme
 
@@ -66,7 +66,7 @@ extend the theme. From version 0.16 on MkDocs implements [theme extension][3],
 an easy way to override parts of a theme without forking and changing the
 main theme.
 
-  [3]: http://www.mkdocs.org/user-guide/styling-your-docs/#using-the-theme_dir
+  [3]: https://www.mkdocs.org/user-guide/styling-your-docs/#using-the-theme-custom_dir
 
 ### Setup and theme structure
 
@@ -167,7 +167,7 @@ The Material theme provides the following template blocks:
 
 For more on this topic refer to the [MkDocs documentation][4]
 
-  [4]: http://www.mkdocs.org/user-guide/styling-your-docs/#overriding-template-blocks
+  [4]: https://www.mkdocs.org/user-guide/styling-your-docs/#overriding-template-blocks
 
 ## Theme development