C++: Improve metadata for GlobalNamespaceClasses.ql.

Author: geoffw0

cpp/ql/src/Architecture/General Namespace-Level Information/GlobalNamespaceClasses.ql

@@ -1,7 +1,8 @@
 /**
  * @name Global namespace classes
  * @description Finds classes that belong to no namespace.
- * @kind table
+ * @kind problem
+ * @problem.severity recommendation
  * @id cpp/architecture/global-namespace-classes
  * @tags maintainability
  *       modularity