Update referenced spec for external-body and add direct link
Update referenced spec for external-body and add direct link

- Change rfc1521 to rfc2046
- Add direct link to https://tools.ietf.org/html/rfc2046#section-5.2.3

Follow-on to: https://github.com/fcrepo/fcrepo-specification/pull/181

fcrepo/fcrepo-specification,fcrepo/fcrepo-specification