[slides][dunemodule] Add file paths for code blocks.

slides/dunemodule.md

@@ -31,6 +31,7 @@ Depends: dumux dune-alugrid dune-foamgrid dune-uggrid
 # Optional build dependencies
 #Suggests:
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/dune.module`</span>
 
 ## Creating a new dune module
 Start the script `duneproject` to initiate creating a new module: