Fix outdated code reference

ExecCreatePartitionPruneState was renamed by commit 297daa9d43, but
this test file didn't get the memo.  Repair.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqFLw=oLX0tP9kcKBmoOExNjDaoAe99dRcxo-GdB9abP9A@mail.gmail.com

src/test/modules/delay_execution/specs/partition-addition.spec

@@ -20,7 +20,7 @@ teardown
 # of which we expect foo1 to be pruned at planning and foo3 at execution.
 # Then we'll block, and by the time the query is actually executed,
 # partition foo2 will also exist.  We expect that not to be scanned.
-# This test is specifically designed to check ExecCreatePartitionPruneState's
+# This test is specifically designed to check CreatePartitionPruneState's
 # code for matching up the partition lists in such cases.
 
 session "s1"