[PATCH] CLEANUP: Fix typos in the dns subsystem

[Joseph Herlant]

Hi,

This patch fixes some typos in the code comments of the dns subsystem.

Note that a typo has been found in the name of the
`prefered_query_type` variable which should be `preferred_query_type`,
but I'm not sure if you're ok with me fixing that too, so I left it
out for now.

The patch is attached, but if you want to view the diff online, you
can check: https://github.com/haproxy/haproxy/compare/master...aerostitch:cleanup_dns_typos

Thanks
Joseph