[slides] add ff-pm slides to the index.md

slides/index.md

@@ -2,7 +2,6 @@
 title: DuMu^x^ course slides
 subtitle: Overview
 ---
-
 **Introduction**
 
 * [Introduction to DuMu^x^](./intro.html)
@@ -32,6 +31,7 @@ subtitle: Overview
 **[Part III]** Advanced exercises, multidomain models
 
 * [Introduction to Multidomain](./multidomain.html)
-* [Free-flow Porous Medium Flow Systems](./coupled_ff-pm.html)
+* [Free-Flow Porous-Medium Flow Systems](./coupled_ff-pm.html)
 * [Discrete Fracture Modeling](./fractures.html)
 * [Biomineralization Modeling](./biomin.html)
+