Use html5.rnc in place of xhtml5.rnc for non-xhtml html files

This change to schemas.xml appears to allow correct detection of rnc schema based upon either the filename or document contents. Without this change, nxml would attempt to apply the xhtml5 schema to plain html5 docs.
Merge pull request #6 from purcell/patch-1

Use html5.rnc in place of xhtml5.rnc for non-xhtml html files