Fix typo in comment
Fix typo in comment

Commit fd31cd265138 renamed the variable to skipping_blocks, but forgot
to update this comment.

Noticed while inspecting code.

lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb