changed heading levels in draft vocabularies section

I made two changes to the draft vocabularies section (see diff [1]) 

(1) Removed the "some observations" section, which has been refactored and moved into the current situation
(2) Changed heading levels, since there was only one subsection "An inventory of existing library linked data resources". In particular:
- Removed "An inventory of existing library linked data resources" as a heading (but used this phrase in describing the side deliverable--is that correct?)
- Added headings for the three types discussed (metadata element sets, value vocabularies, and datasets)

Please make any corrections!

-Jodi

[1] http://www.w3.org/2005/Incubator/lld/wiki/index.php?title=Draft_Vocabularies_Datasets_Section&diff=5707&oldid=5672