Update mediawiki http to https links in CategoryTree extension

This patch updates the old http links of mediawiki to https in
the CategoryTree extension (in the README specifically).

Bug: T189687
Change-Id: I0872762f9adff90884cb89e64918574b1266fa79

README

@@ -15,4 +15,4 @@ they are mentioned in each file individually. Also thanks to Tim Starling
 for his contributions.
 
 Instructions on installing and using this extension are available at
-<http://www.mediawiki.org/wiki/Extension:CategoryTree>
+<https://www.mediawiki.org/wiki/Extension:CategoryTree>