Fix link to global search design document

The link to the pdf copy of the global search design document is broken
patch it up so it is downloadable from the link.

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

content/interface_discovery.md

@@ -16,9 +16,8 @@ to the "API contract" for this set of functionality as an
 
 ## Use cases
 
-  - A global search
-    ([<File:apertis-global-search-design-0.3.2.pdf>](/apertis-global-search-design-0.3.2.pdf))
-    user interface requires a list of agents that can act as "Auxiliary
+  - A [global search user interface](/images/apertis-global-search-design-0.3.2.pdf)
+    requires a list of agents that can act as "Auxiliary
     Sources" (see §6.2 in the Global Search design document). For
     example, a Spotify client might register itself as a search provider
     so that searching for a term in a global search will find artists or
@@ -304,4 +303,4 @@ The service providing this D-Bus API (for example Didcot) should be a
 platform component. It is considered to be a trusted component for the
 purposes of security between app-bundles: it must reveal public
 interface implementations to other app-bundles, but must only reveal
-non-public interface implementations to trusted platform components.
\ No newline at end of file
+non-public interface implementations to trusted platform components.