Fix link to Minz docs in 05_Extensions.md (#8229)

The link to the Minz docs works on GitHub but 404's on the [live site](https://freshrss.github.io/FreshRSS/en/developers/03_Backend/05_Extensions.html). Replacing the absolute path with a relative path should correct the 404.

docs/en/developers/03_Backend/05_Extensions.md

@@ -24,7 +24,7 @@ Note: it is quite conceivable that the functionalities of an extension can later
 
 ## Minz Framework
 
-see [Minz documentation](/docs/en/developers/Minz/index.md)
+see [Minz documentation](../Minz/index.md)
 
 ## Write an extension for FreshRSS