Documentation: Fix the address of the linuxppc-dev mailing list

This list was moved many years ago.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240503121012.3ba5000b@canb.auug.org.au