Tweak path to multimedia.md due to naming clash

Both the developer and designs documentation contain a file named
"multimedia.md`. Tweak the path used in the documentation to remove the
ambiguity.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/wip/filesystem_layout.md

@@ -485,7 +485,7 @@ moved from `/General/applications` into `/General`.
 
   - [Applications design document]( {{< ref "applications.md" >}} ) (v0.5.4
     used)
-  - [Multimedia design document]( {{< ref "multimedia.md" >}} ) (v0.5.4
+  - [Multimedia design document]( {{< ref "designs/multimedia.md" >}} ) (v0.5.4
     used)
   - [Security design document]( {{< ref "security.md" >}} ) (v1.1.3 used)
   - [System Update & Rollback design document]( {{< ref "system-updates-and-rollback.md" >}} ) (v1.6.2 used)