Fixed off-by-one error in Xom spell selection.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2922 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on November 28, 2007
OKPS67MLJ34EKGNZOBBD74DSQYUNXO7XGBHZO22QXWWZBA6A2WXQC
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