Fix link to image at docs

docs/Features/Allow-private-boards-only.md

@@ -1,3 +1,3 @@
 [Wekan v5.55 and newer](https://github.com/wekan/wekan/blob/main/CHANGELOG.md#v555-2021-08-31-wekan-release)
 
-<img src="https://wekan.fi/allow-private-boards-only.png" width="60%" alt="Allow private boards only" />
+<img src="allow-private-boards-only.png" width="60%" alt="Allow private boards only" />