fixing broken image ref

See docs state <img..> tags in .md https://docusaurus.io/docs/static-assets#in-markdown

Author: mccrackend

docs/docs/getting-started/admin-gui.md

@@ -10,4 +10,4 @@ This plugin ships with a React app which can be accessed from the settings page
 **Pro tip:**
 By clicking on one of the items in the diff table you can see the exact difference between sync dir and database in a git-style diff viewer.
 
-<img src="/img/assets/admin-diff-viewer.png" alt="Config diff in admin" />
+![Config diff in admin](/img/assets/admin-diff-viewer.png)