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 <louis@kragniz.eu>

README.md

@@ -50,7 +50,7 @@ netdev | Exposes network interface statistics such as bytes transferred. | Darwi
 netstat | Exposes network statistics from `/proc/net/netstat`. This is the same information as `netstat -s`. | Linux
 nfs | Exposes NFS client statistics from `/proc/net/rpc/nfs`. This is the same information as `nfsstat -c`. | Linux
 nfsd | Exposes NFS kernel server statistics from `/proc/net/rpc/nfsd`. This is the same information as `nfsstat -s`. | Linux
-pressure | Exposes pressure stall statistics from `/proc/pressure/`. | Linux (kernel 4.20+ and/or [CONFIG\_PSI](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/accounting/psi.txt))
+pressure | Exposes pressure stall statistics from `/proc/pressure/`. | Linux (kernel 4.20+ and/or [CONFIG\_PSI](https://www.kernel.org/doc/html/latest/accounting/psi.html))
 rapl | Exposes various statistics from `/sys/class/powercap`. | Linux
 schedstat | Exposes task scheduler statistics from `/proc/schedstat`. | Linux
 sockstat | Exposes various statistics from `/proc/net/sockstat`. | Linux