fix: updated null-ls format on save setup instructions for latest neovim

[Clish254]

Fixes #18 Updated null-ls setup instructions for latest neovim to fix the error below which is mentioned in issue #18

Error detected while processing BufWritePost Autocommands for "<buffer=3>":
E5108: Error executing lua [string ":lua"]:1: attempt to call field 'formatting' (a nil value)
stack traceback:
        [string ":lua"]:1: in main chunk

The code is from null-ls docs here