Change supportedFormat to supportedFormats for better compatibility

Change supportedFormat to supportedFormats for better compatibility between DescribeCoverage and GetCapabilities.  The XML is itself inconsistent, which was probably why this problem went unnoticed.  

It makes sense to use the same internal identifier for the same information to simplify parsing the resulting OpenLayers objects.  That way the user is trying to extract the supportedFormats data, they can do it with either a DescribeCoverage or a GetCapabilities object.