"Public Domain" is not the name of a license, the title of "Unlicense" should be something else

"Public Domain Dedication (Unlicense)" is a better title for the Unlicense than just "Public Domain". The "public domain" per se is not a license, it's... a... domain.

Also changes the permalink from "public-domain" to "unlicense" to distinguish it from other methods of making a public domain dedication. This brings it in line with other licenses for which the permalink is the name of the license not some concept (like "/licenses/opensource").