Add missing "format" facet in Date type

[jstoiko]

The "Date" type definition section mentions the "additional facet format" multiple times however it is not formated as a table like in the other sections

Proposed Change

Reformat the sentence below as a table:

The additional facet format MUST be available only when the type equals datetime, and the value MUST be either rfc3339 or rfc2616. Any other values are invalid.

E.g.

Facet	Description
format?	The additional facet format MUST be available only when the type equals datetime, and the value MUST be either rfc3339 or rfc2616. Any other values are invalid.