fix: spaces to dash (instead of underscore) in fragment id of markdown links (EbookFoundation#7031)

* also spaces to dash (instead of underscore)

Author: davorpa

courses/free-courses-si.md

@@ -1,6 +1,6 @@
 ### Index
 
-* [ASP.NET Core](#aspnet_core)
+* [ASP.NET Core](#aspnet-core)
 * [HTML and CSS](#html-and-css)
 * [Java](#java)
 * [JavaScript](#javascript)