README references `NewLeoRSFF8Codec` which doesn't exist

[rootulp]

Context

rsmt2d/README.md

Line 24 in 2557620

codec := rsmt2d.NewLeoRSFF8Codec()

Problem

#136 included a breaking change that removed NewLeoRSFF8Codec() and introduced NewLeoRSCodec()

Proposal

Update the README