nixos/tomcat: fix eval error introduced by #40657

(cherry picked from commit 93cbb9b72fed5076d4d0038f14625be4debde255)

nixos/modules/services/web-servers/tomcat.nix

···
 
       webapps = mkOption {
         type = types.listOf types.package;
-        default = [ tomcat85.webapps ];
-        defaultText = "[ tomcat85.webapps ]";
+        default = [ tomcat.webapps ];
+        defaultText = "[ pkgs.tomcat85.webapps ]";
         description = "List containing WAR files or directories with WAR files which are web applications to be deployed on Tomcat";
       };