x86/mm/pkeys: Fix typo in Documentation/x86/protection-keys.txt

Replace PKEY_DENY_WRITE with PKEY_DISABLE_WRITE,
to match the source code.

Signed-off-by: Wang Kai <morgan.wang@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave.hansen@intel.com
Cc: dave.hansen@linux.intel.com
Cc: linux-doc@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>