Sponsorship Page Language

Hello,

When looking at the sponsorship page (http://www.postgresql.org/about/sponsors/) I noticed a few things regarding the language that I've attempted to clean up with this patch.

The first regards the ordering of the sponsors - they are ordered randomly, not alphabetically, so I corrected the language up top.

The second has to do with the language on the bottom of the page discussing the sponsorship levels and selection process.  I reworded the sentences to more clearly state the purpose of the sponsorship levels are and how they are selected, maintaing a link to the guidelines on the wiki.

Best,

Jonathan