Update summary.rst

I'm confused.  I couldn't find a section called "Executive Summary" in: https://www.opentechfund.org/submit/guide#sections .

I did see:

"Proposal Summary":  400 - 1000 words

I wc'd the "Executive Summary" it containes 1167 words.  If it's the "Proposal Summary" then I recommend we remove >= 167 words, and s/Executive/Proposal in the title.