Fixed minor documentation spelling mistake
Fixed minor documentation spelling mistake

Signed-off-by: Dan Kubb <c78015a524f662a52b2d107a16c9e5d6e7c7795b@gmail.com>

tpitale/dm-core,ar-dm/ardm-core,datamapper/dm-core,engineyard/dm-core,tillsc/dm-core,alloy/dm-core