Docs: remove Squid prefix from ntlm_auth example paths

Docs: remove Squid prefix from ntlm_auth example paths

There is no longer a helper called ntlm_auth shipped with Squid and the
example configs are supposed to be referring to the Samba helper now
as demonstrated by the command line options.

/usr/bin/ is probably not the only location where Samba installs its
helper, but is easily recognised and better than prefixing with a custom
Squid-specific path from --prefix.