Fix inconsistency in comments atop ExecParallelEstimate.

When this code was initially introduced in commit d1b7c1ff, the structure
used was SharedPlanStateInstrumentation, but later when it got changed to
Instrumentation structure in commit b287df70, we forgot to update the
comment.

Reported-by: Wu Fei
Author: Wu Fei
Reviewed-by: Amit Kapila
Backpatch-through: 9.6
Discussion: https://postgr.es/m/52E6E0843B9D774C8C73D6CF64402F0562215EB2@G08CNEXMBPEKD02.g08.fujitsu.local

src/backend/executor/execParallel.c

@@ -219,7 +219,7 @@ ExecSerializePlan(Plan *plan, EState *estate)
  * &pcxt->estimator.
  *
  * While we're at it, count the number of PlanState nodes in the tree, so
- * we know how many SharedPlanStateInstrumentation structures we need.
+ * we know how many Instrumentation structures we need.
  */
 static bool
 ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)