Fixed grammar in abstract

"a applicant" changed to "an applicant"
Added missing punctuation

"." was missing at the end of a sentence
Fixed reference to invalid section

A reference to "{#validation}" has been replaced with "{{identifier-validation-challenges}}"
Typo fixes
Fixed sentence fragment
Added interval to example "defer" response
Fix typo
un-HTML escape in code blocks
Standardize quotation of message types

Missing quotations are now added to message types
Merge branch 'master' of https://github.com/lezed1/acme-spec into lezed1-master