Changed <span> to <seg>

This issue came up when trying to validate against the Crosswire
TEI/OSIS schema, and Chris L. pointed out that <span> in TEI does not
mean the same thing as <span> in html. He recommended using <seg>
instead. For a definition of this element, see
http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-seg.html.