docs: fix migration guide title (#2425)

Signed-off-by: Ludovic Ortega <ludovic.ortega@adminafk.fr>

Author: M0NsTeRRR

docs/migration-guide.mdx

@@ -200,14 +200,15 @@ Summary of changes :
   </TabItem>
 </Tabs>
 
-### Nix (Third-party installation methods)
+## Third-party installation methods
+### Nix
 
 Waiting for https://github.com/NixOS/nixpkgs/pull/450096 and https://github.com/NixOS/nixpkgs/pull/450093
 
-### AUR (Third-party installation methods)
+### AUR
 
 See https://aur.archlinux.org/packages/seerr
 
-### TrueNAS (Third-party installation methods)
+### TrueNAS
 
 Waiting for https://github.com/truenas/apps/issues/3374