Replace "back-slash" with "backslash" in docs.

I kept "back slash" when explicitely used in
comparison with "forward slash".

Backport of r965792 from trunk and of r965799
from 2.2.x.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@965803 13f79535-47bb-0310-9956-ffa450edef68