Update example in README from j to z

[jgonera]

I spent 5 minutes trying to figure out why j didn't work.

[jgonera]

Is j really better?

[ernstki]

@jgonera Yes, most definitely, because j is in the home row, it's mnemonic (jump!), and it's easier for recent converts who previously used autojump and because of years of muscle-memory will continue to reach for j regardless. ;)

[jgonera]

@ernstki right but it doesn't seem to be the default in this project. Or am I missing something?