nixos/samba: fix pam service name typo (#28049)

The PAM service name used before this commit was "sambda", with an
extra 'd'. For some reason I don't quite fully understand this typo
prevents GDM from starting. This change fixes that as tested in VMs
built using "nixos-rebuild -I nixpkgs=<mypkgs> build-vm".

nixos/modules/services/network-filesystems/samba.nix

···
           };
         };
 
-        security.pam.services.sambda = {};
+        security.pam.services.samba = {};
 
       })
     ];