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

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

Backport of r965792 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@965799 13f79535-47bb-0310-9956-ffa450edef68