mm,thp: fix spellos in describing __HAVE_ARCH_FLUSH_PMD_TLB_RANGE

[akpm@linux-foundation.org: s/threshhold/threshold/]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>