Fix setrefs.c comment properly.

The "typo" alleged in commit 1e460d4bd was actually a comment that was
correct when written, but I missed updating it in commit b5282aa89.
Use a slightly less specific (and hopefully more future-proof) description
of what is collected.  Back-patch to 9.2 where that commit appeared, and
revert the comment to its then-entirely-correct state before that.

src/backend/optimizer/plan/setrefs.c

@@ -1243,7 +1243,7 @@ copyVar(Var *var)
  * This is code that is common to all variants of expression-fixing.
  * We must look up operator opcode info for OpExpr and related nodes,
  * add OIDs from regclass Const nodes into root->glob->relationOids, and
- * add catalog OIDs for user-defined functions into root->glob->invalItems.
+ * add PlanInvalItems for user-defined functions into root->glob->invalItems.
  * We also fill in column index lists for GROUPING() expressions.
  *
  * We assume it's okay to update opcode info in-place.  So this could possibly