fix grammar in readme.txt
fix grammar in readme.txt

bgould/avian,minor-jason/avian,lwahlmeier/avian,bigfatbrowncat/avian-pack.avian,lwahlmeier/avian,lostdj/avian,getlantern/avian,badlogic/avian,lwahlmeier/avian,joshuawarner32/avian,badlogic/avian,badlogic/avian,bgould/avian,dicej/avian,dicej/avian,marcinolawski/avian,lostdj/avian,bigfatbrowncat/avian-pack.avian,getlantern/avian,marcinolawski/avian,MaartenR/avian,bgould/avian,marcinolawski/avian,bigfatbrowncat/avian-pack.avian,getlantern/avian,minor-jason/avian,minor-jason/avian,ucdseniordesign/avian,MaartenR/avian,ucdseniordesign/avian,dicej/avian,lwahlmeier/avian,dicej/avian,joshuawarner32/avian,MaartenR/avian,badlogic/avian,minor-jason/avian,getlantern/avian,lostdj/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,joshuawarner32/avian,MaartenR/avian,bgould/avian,ucdseniordesign/avian,marcinolawski/avian,joshuawarner32/avian,ucdseniordesign/avian