powerpc/mm/hash: Fix off-by-one in comment about kernel contexts ids

Michal Suchánek noticed a comment in book3s/64/mmu-hash.h about the context ids
we use for the kernel was inconsistent with the code and other comments in the
same file.

It should read 1-4 not 1-5.

While we're touching it, update "address" to "addresses" which makes more sense
as it's referring to more than one address below.

Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>