docs: fixed broken url

Signed-off-by: Alexander Piskun <bigcat88@icloud.com>

docs/developer.rst

@@ -4,7 +4,7 @@ App Developer Guide
 ===================
 
 Note: Documentation and a tutorial about App Development itself (vs "app development and the App Store") is found at
-`https://docs.nextcloud.com/server/stable/developer_manual/app/index.html <https://docs.nextcloud.com/server/stable/developer_manual/app/index.html>`_.
+`https://docs.nextcloud.com/server/stable/developer_manual/app_development/index.html <https://docs.nextcloud.com/server/stable/developer_manual/app_development/index.html>`_.
 
 Most of today's developers publish their source code on GitHub, BitBucket, GitLab or on their own GitLab instance. These tools typically also provide a way to release new versions based on Git tags or by uploading custom archives.