readme: fix slack link
readme: fix slack link

Signed-off-by: Louis Taylor <d82ece8d514aca7e24d3fc11fbb8dada57f2966c@kragniz.eu>

rook/nfs,rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook,rook/rook,rook/nfs