Fix warning: "Use midRef() instead of mid()"

[-Wclazy-qstring-ref]

Change-Id: If8a0844b39377feb3772542559655854a92b93cd
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmakeprojectmanager/customwidgetwizard/classdefinition.cpp

@@ -79,7 +79,7 @@ static inline QString xmlFromClassName(const QString &name)
     if (!name.isEmpty()) {
         rc += name.left(1).toLower();
         if (name.size() > 1)
-            rc += name.mid(1);
+            rc += name.midRef(1);
     }
     rc += QLatin1String("\">\n</widget>\n");
     return rc;