Update configuration.rst
Update configuration.rst

I really bumped my head hard on this issue https://github.com/mapproxy/mapproxy/issues/490 which requires to use the older layer syntax. Further the deprecated example uses source: (singular) for sources key which further makes even trying to use the deprecated syntax by following the example given hard. This PR tries to make things a little clearer.
mapproxy/mapproxy,mapproxy/mapproxy