Hi EMOS,
The Resources.xml file is used only during the installation of the plugin. After that you need to change the resources from the administration.
Go to Administration -> Configuration -> Languages, on the row of your working language you should have
View string resources link. If you click on it you should see a grid containing all the resources for this language. Search for resource named
sevenspikes.nopquicktabs.client.tabs.contactus.tabtitle (you could use the grid filter option to locate the resource faster). When you find it you could update it or delete it and restart the application and the system will automatically load it from the Resources.xml file.
You could see this
video as it explains how to search and change resources.
Hope this helps!
Regards