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>

arch/powerpc/include/asm/book3s/64/mmu-hash.h

@@ -494,7 +494,7 @@ extern void slb_set_size(u16 size);
  *
  * For user processes max context id is limited to MAX_USER_CONTEXT.
 
- * For kernel space, we use context ids 1-5 to map address as below:
+ * For kernel space, we use context ids 1-4 to map addresses as below:
  * NOTE: each context only support 64TB now.
  * 0x00001 -  [ 0xc000000000000000 - 0xc0003fffffffffff ]
  * 0x00002 -  [ 0xd000000000000000 - 0xd0003fffffffffff ]