Help: Use override consistently

clang-tidy fixes from modernize-use-override check.

Change-Id: I3aec6d7ab1e8f088b745d2d9730e3d49ced42031
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/localhelpmanager.h

@@ -58,7 +58,7 @@ public:
     };
 
     LocalHelpManager(QObject *parent = nullptr);
-    ~LocalHelpManager();
+    ~LocalHelpManager() override;
 
     static LocalHelpManager *instance();