Fix link to developers docs in readme.md (#3712)

README.md

@@ -37,7 +37,7 @@ FreshRSS comes with absolutely no warranty.
 
 * [User documentation](https://freshrss.github.io/FreshRSS/en/users/02_First_steps.html), where you can discover all the possibilities offered by FreshRSS
 * [Administrator documentation](https://freshrss.github.io/FreshRSS/en/admins/01_Index.html) for detailed installation and maintenance related tasks
-* [Developer documentation](https://freshrss.github.io/FreshRSS/en/developers/01_First_steps.html) to guide you in the source code of FreshRSS and to help you if you want to contribute
+* [Developer documentation](https://freshrss.github.io/FreshRSS/en/developers/01_Index.html) to guide you in the source code of FreshRSS and to help you if you want to contribute
 * [Contributor guidelines](https://freshrss.github.io/FreshRSS/en/contributing.html) for those who want to help improve FreshRSS
 
 # Requirements