Rename ABIL_SUMMON_DEMONS to ABIL_SUMMON_DEMON, for consistency.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5559 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on June 7, 2008
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