maint: fix comment typos

maint: fix comment typos

Using http://github.com/lyda/misspell-check, massage its
output into sed commands to perform the suggested changes.
Initially, I filtered out the THRU->Through changes, because
that failed to retain capitalization in the grammar token.
Instead, do this manually, beforehand:

sed -i s/THRU/THROUGH/ tests/existing.at
git ls-files|misspellings -f -|perl -nl \
  -e '/^(.*?)\[(\d+)\]: (\w+) -> "(.*?)"$/ or next;' \
  -e '($file,$n,$l,$r)=($1,$2,$3,$4); $q="'\''"; $r=~s/$q/$q\\$q$q/g;'\
  -e 'print "sed -i $q${n}s!$l!$r!$q $file"'|bash