Remove extra word from comment.

David Rowley, who also was the primary author of the patch that
added this function; the attribution in my previous commit,
84940644, was incorrect due to
sloppiness on my part.

Discussion: http://postgr.es/m/CAKJS1f_0iSiLQsf_c06AzOWAc3eS6ePjjVQFpcFv3W-O5aktnQ@mail.gmail.com

src/backend/nodes/bitmapset.c

@@ -789,8 +789,8 @@ bms_add_members(Bitmapset *a, const Bitmapset *b)
  *		Add members in the range of 'lower' to 'upper' to the set.
  *
  * Note this could also be done by calling bms_add_member in a loop, however,
- * using this function will be faster when the range is large as we work with
- * at the bitmapword level rather than at bit level.
+ * using this function will be faster when the range is large as we work at
+ * the bitmapword level rather than at bit level.
  */
 Bitmapset *
 bms_add_range(Bitmapset *a, int lower, int upper)