typos in draft

I noticed a couple of minor typos in the latest version of the spec, edited them
and highlighted in red,
attached, just so they don't get missed.

end of 3.1 *as* children
end of 3.3 remove duplicated line
5.1 minOccurs misspelled
5.5 child of *an* EncryptedData or EncryptedKey

This draft is looking really good, and the algorithm section is very clear.