Fix typo in comment.

Etsuro Fujita

Discussion: http://postgr.es/m/5A0D7C3D.80803@lab.ntt.co.jp

src/backend/catalog/partition.c

@@ -2686,7 +2686,7 @@ qsort_partition_rbound_cmp(const void *a, const void *b, void *arg)
 /*
  * partition_rbound_cmp
  *
- * Return for two range bounds whether the 1st one (specified in datum1,
+ * Return for two range bounds whether the 1st one (specified in datums1,
  * kind1, and lower1) is <, =, or > the bound specified in *b2.
  *
  * Note that if the values of the two range bounds compare equal, then we take