simple-markdown: Fix paragraph end delimeter

Summary:
Paragraphs now allow spaces in the newline(s) before their
final newline. Apparently this is something people actually
write.

Test Plan:
`make test`

Auditors: emily
simple-markdown: output actual newlines not spaces

Summary:
For newlines, it now outputs '\n' instead of ' '.
This mostly makes diffing easier but also makes more
sense, given that newlines in text remain '\n's.

Test Plan:
 #yolo

Auditors: emily