fixed incorrect documentation, webappconfig should be web app

in the example configuration for maven the configuration element should be 'webApp' not 'webAppConfig'

I've fixed this for the override xml as this is the one that I was using when I found the error.

It also looks like the same error is in two other files, but I have not modified them as I have not tested if  these are broken as well:
https://github.com/jetty-project/jetty-documentation/blob/063695ec030e83cbb3d06e4182334b4c19a77e44/src/docbkx/reference/jetty-xml/webdefault-xml.xml
https://github.com/jetty-project/jetty-documentation/blob/0815a8856020fbad1d085f0fd6c802a6cdd8a9e6/src/docbkx/administration/sessions/using-persistent-sessions.xml

I can update these as well if you want but I have not tested to confirm correctness