phpmd: Fix meta.broken

In commit e76ccc6b27 ("php.extensions: Add missing meta info",
2021-06-08), Nix expressions for PHP packages were refactored and in
case of phpmd, the value of meta.broken was negated (likely by
mistake). The result was that phpmd is marked as broken, but it seems
to work well, at least for my use case.

Here, we correct the mentioned commit by negating meta.broken again.

pkgs/development/php-packages/phpmd/default.nix

@@ -27,6 +27,6 @@ mkDerivation {
     license = licenses.bsd3;
     homepage = "https://phpmd.org/";
     maintainers = teams.php.members;
-    broken = versionAtLeast php.version "7.4";
+    broken = versionOlder php.version "7.4";
   };
 }