readme: fix link to PSI documentation (#1864)
readme: fix link to PSI documentation (#1864)

The txt was changed to rst:

    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/accounting/psi.rst

But it's probably better to link to the rendered docs, since the link
should be more stable.

Signed-off-by: Louis Taylor <d82ece8d514aca7e24d3fc11fbb8dada57f2966c@kragniz.eu>
prometheus/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,siavashs/node_exporter