docs: Fix interface prefix for D-Bus docs

The --interface-prefix= gtk-doc argument was being passed a stringified
array instead of a simple string.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/644

docs/reference/shell/meson.build

@@ -28,7 +28,7 @@ foreach iface : ifaces
     output: 'doc-gen-' + iface[1],
     command: [
       'gdbus-codegen',
-      '--interface-prefix=@0@.'.format(iface),
+      '--interface-prefix=@0@.'.format(iface[0]),
       '--generate-docbook', 'doc-gen',
       '--output-directory', '@OUTDIR@',
       '@INPUT@'