Update NixOS module URL (#13818)

* Update NixOS module URL

* Create 13818.doc

changelog.d/13818.doc

@@ -0,0 +1 @@
+Update URL for the NixOS module for Synapse.

docs/setup/installation.md

@@ -181,7 +181,7 @@ doas pkg_add synapse
 #### NixOS
 
 Robin Lambertz has packaged Synapse for NixOS at:
-<https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/misc/matrix-synapse.nix>
+<https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/matrix/synapse.nix>
 
 
 ### Installing as a Python module from PyPI