Update pnas.csl

Solves https://forums.zotero.org/discussion/40717/style-error-proceedings-of-the-national-academy-of-sciences-of-the-united-states-of-america/#Item_0
and another problem with interpunctation in chapters, see example citation in style repo.
Merge pull request #1144 from zuphilip/patch-1

Update pnas.csl