nixos/matrix-authentication-service: escape CREDENTIALS_DIRECTORY correctly

The `''` is needed to make Nix not interpret it as variable. However, it
would now render in the manual as `${CREDENTIALS_DIRECTORY}`. If
somebody would copy that, he'd get an error because that's a variable
substitution then.

Adding another backslash such that the code rendered in the manual is
correct.

nixos/modules/services/matrix/matrix-authentication-service.nix

@@ -385,7 +385,7 @@ in
         For example :
         services.matrix-authentication-service.credentials."synapse-secret" = "/run/agenix/synapse-shared";
         services.matrix-authentication-service.settings.matrix.secret_file =
-          "''${CREDENTIALS_DIRECTORY}/synapse-secret";
+          "\''${CREDENTIALS_DIRECTORY}/synapse-secret";
       '';
     };
   };