Make text consistent with example.
The Gettysburg Address was given two different titles:
 * Soldiers' National Cemetery Dedication
 * Dedication of the Soldiers' National Cemetery
This commit makes them consistent.
If we aren't going to call it the "Gettysburg Address", then we might as well
use Abraham Lincoln's preferred title, from the authoritative Bliss text:
"Address delivered at the dedication of the Cemetery at Gettysburg"
Merge pull request #317 from mernst/upstream-gettysburg

Make text consistent with example.