Example for`SplitGrantR` `bufs` is incorrect

[andresv]

split_read() was recently added, so example for bufs() is probably copy-pasted from other method and therefore incorrect. https://github.com/jamesmunns/bbqueue/blob/479eb3d5884c9a7459f630b333ad6f21efb3cec0/core/src/bbbuffer.rs#L1078-L1109