Fix docs link to dev themes (#3746)

* link fixed

* fix2

docs/en/admins/11_Themes.md

@@ -14,4 +14,4 @@ or a similar method to add their theme to their FreshRSS instance.
 
 # Creating themes
 
-Information on creating themes can be found in [the developer documentation.](/docs/en/developers/04_Frontend/02_Design)
+Information on creating themes can be found in [the developer documentation.](../developers/04_Frontend/02_Design.md)