Fix spelling error in rarely used pre-release API

MetadataTokens.ExportedTypeHandle was misspelled with two o's.

This is of course a breaking change, but this API is still in
pre-release. Furthermore, there are no usages at all of this API
in any partner code that I can find and it is unlikely to be used
yet elsewhere. The API is in the Ecma335 sub-namespace for getting
handles from raw metadata tokens, but most consumers would find
ExportedTypes via the main token-agnostic API.