update readme

README.md

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 # mhcflurry
-[MHC I](https://en.wikipedia.org/wiki/Major_histocompatibility_complex) ligand
+[MHC I](https://en.wikipedia.org/wiki/MHC_class_I) ligand
 prediction package with competitive accuracy and a fast and 
 [documented](http://www.hammerlab.org/mhcflurry/) implementation.