Correct typo in awful.client.restore

This corrects a type in awful.client.restore which caused it not to restore any
client at all  if all clients are minimized.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client.lua.in

@@ -644,7 +644,7 @@ end
 -- @param s The screen to use.
 -- @return True if some client was restored.
 function restore(s)
-    local s = s or (capi.client.focus and capi.client.focus.screen) or mouse.screen
+    local s = s or (capi.client.focus and capi.client.focus.screen) or capi.mouse.screen
     local cls = capi.client.get(s)
     local tags = tag.selectedlist(s)
     local mcls = {}