replace Eventual IO with tokio-core in the README
replace Eventual IO with tokio-core in the README

asomers/mio,carllerche/mio,fuchsia-mirror/third_party-mio,carllerche/mio,asomers/mio,murarth/mio,mneumann/mio,mneumann/mio,fuchsia-mirror/third_party-mio