DOC: Update README to use "https" links

Co-authored-by: Lee Newberg <lee.newberg@kitware.com>

Author: jcfr

README.md

@@ -15,7 +15,7 @@ A website referred to as the [extensions catalog][] provides Slicer users with a
 The following diagram depicts how `extensions catalog`, `extensions server`, [CDash][] and the
 slicer factory interact.
 
-<img width="80%" src="http://www.slicer.org/slicerWiki/images/a/ab/Extensions-Index-to-Catalog-cycle.png"/>
+<img width="80%" src="https://www.slicer.org/slicerWiki/images/a/ab/Extensions-Index-to-Catalog-cycle.png"/>
 
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