Fix minor typo in nodeIncrementalSort.c.

Author: Vignesh C
Reviewed-by: James Coleman
Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/CALDaNm0WjZqRvdeL59ZfYH0o4mLbKQ23jm-bnjXcFzgpANx55g@mail.gmail.com

src/backend/executor/nodeIncrementalSort.c

@@ -65,8 +65,8 @@
  *	into the second mode if we believe it's beneficial.
  *
  *	Sorting incrementally can potentially use less memory, avoid fetching
- *	and sorting all tuples in the the dataset, and begin returning tuples
- *	before the entire result set is available.
+ *	and sorting all tuples in the dataset, and begin returning tuples before
+ *	the entire result set is available.
  *
  *	The hybrid mode approach allows us to optimize for both very small
  *	groups (where the overhead of a new tuplesort is high) and very	large