pgsql: Update comment for IndexInfo.ii_NullsNotDistinct

Update comment for IndexInfo.ii_NullsNotDistinct

Commit 7a7b3e11e61 added the ii_NullsNotDistinct field, but the
comment was not updated.

Author: Japin Li <japinli@hotmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/ME0P300MB04453E6C7EA635F0ECF41BFCB6832%40ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/779f24c0b7be5d66504a30c6d15ab0430bdf59e2

Modified Files
--------------
src/include/nodes/execnodes.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)