fix some typos in the ab man page

PR:   17108
Submitted by:  Eric S. Raymond <esr@thyrsus.com>
Reviewed by:   Jeff Trawick (who fixed a couple of mispellings of "probably")


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@98718 13f79535-47bb-0310-9956-ffa450edef68