Comment fixes in create_grouping_paths() add_paths_to_append_rel()

Hi
Here are two patches fixing comments.

0001
A comment in add_paths_to_append_rel() mentions the number of paths
instead of number of workers by mistake. Fixed it.

0002
Commit b5635948ab165b6070e7d05d111f966e07570d81 added a single
grouping_sets_data argument instead of two lists related to grouping
sets to create_grouping_paths(), but forgot to update the prologue of
the function. Fixed it.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company