include/linux/highmem.h: fix duplicated words in a comment

Change the doubled word "is" in a comment to "it is".
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/ad605959-0083-4794-8d31-6b073300dd6f@infradead.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/highmem.h

@@ -73,7 +73,7 @@ static inline void kunmap(struct page *page)
  * no global lock is needed and because the kmap code must perform a global TLB
  * invalidation when the kmap pool wraps.
  *
- * However when holding an atomic kmap is is not legal to sleep, so atomic
+ * However when holding an atomic kmap it is not legal to sleep, so atomic
  * kmaps are appropriate for short, tight code paths only.
  *
  * The use of kmap_atomic/kunmap_atomic is discouraged - kmap/kunmap