[PATCH] CLEANUP: fix typos in the comments of the Makefile

[Joseph Herlant]

Hi,

This is not user-visible issues, just a cleanup of comments.

As the Makefile not really in a given unit of code and is not really
documentation, I created a separated patch for it. let me know if
you'd rather have me group it with another patch.

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_makefile_typos

Thanks,
Joseph