Fix randart books being much too cheap. (Yes, this was actually a bug.)

A randart book's rarity is now defined as the average of its three rarest spells.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9101 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on February 16, 2009
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