x86: fix typo in address space documentation

Fix a trivial typo in Documentation/x86/x86_64/mm.txt.

[ Impact: documentation only ]
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Rik van Riel <riel@redhat.com>

Documentation/x86/x86_64/mm.txt

@@ -6,7 +6,7 @@ Virtual memory map with 4 level page tables:
 0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm
 hole caused by [48:63] sign extension
 ffff800000000000 - ffff80ffffffffff (=40 bits) guard hole
-ffff880000000000 - ffffc8ffffffffff (=64 TB) direct mapping of all phys. memory
+ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. memory
 ffffc80000000000 - ffffc8ffffffffff (=40 bits) hole
 ffffc90000000000 - ffffe8ffffffffff (=45 bits) vmalloc/ioremap space
 ffffe90000000000 - ffffe9ffffffffff (=40 bits) hole