Fix typo in docs/datamodel and make style improvements

1. s/heirarchy/hierarchy/
2. Phrases that lead up to a list should end with a colon
3. Simplify the sentence leading up to the RDFa Lite example
4. Use an ordered list for the steps to convert microdata into RDFa Lite
5. Update the "last updated" date (fwiw)

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #20 from dbs/datamodel_docs

Fix typo in docs/datamodel and make style improvements