Small change to section 2.7

["Piazza, Rich" <rpiazza@mitre.org>]

While addressing issue 185, the following text was added by the editors to section 2.7 about the values for the hashes dictionary.

 

Dictionary keys MUST be unique in each hashes property, MUST be in ASCII, and are limited to the characters a-z (lowercase ASCII), A-Z (uppercase ASCII), numerals 0-9, hyphen (-), and underscore (_). Dictionary keys MUST have a minimum length of 3 ASCII characters and MUST be no longer than 250 ASCII characters in length. The value MUST be a string in the appropriate format defined by the hash type indicated in the dictionary key.