README: Consistently refer to OOUI as library
README: Consistently refer to OOUI as library

Bug: T179111
Change-Id: Ib8e33498b1df3f5bbcc4b0c02e4eadefb2f5a997

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui