Updated ldap.md
Updated ldap.md

Add mapping of `email` field to `mail` value in LDAP.  Also remove duplicate entries of such mapping information.
snazy2000/netbox,digitalocean/netbox,snazy2000/netbox,snazy2000/netbox,digitalocean/netbox,digitalocean/netbox,lampwins/netbox,snazy2000/netbox,lampwins/netbox,digitalocean/netbox,lampwins/netbox,lampwins/netbox