Update how-to-retrieve-the-number-of-slides-in-a-presentation-document.md

Missed an http link -> covert to https

Author: scottmcarthur

docs/how-to-retrieve-the-number-of-slides-in-a-presentation-document.md

@@ -23,7 +23,7 @@ loading the document into Microsoft PowerPoint. It contains an example
 **RetrieveNumberOfSlides** method to illustrate
 this task.
 
-To use the sample code in this topic, you must install the [Open XML SDK 2.5](http://www.microsoft.com/download/details.aspx?id=30425). You
+To use the sample code in this topic, you must install the [Open XML SDK 2.5](https://www.microsoft.com/download/details.aspx?id=30425). You
 must explicitly reference the following assemblies in your project:
 
 -   WindowsBase