Fixing a trivial typo in comment in rewriteManip.c

While I was looking on some patch, I happened to notice that
there's a trivial typo in rewriteManip.c

> a ConvertRowTypeExpr to map back to the rowtype expected by the expression.

The correct name for "ConvertRowTypeExpr" is "ConvertRowtypeExpr"

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/src/backend/rewrite/rewriteManip.c b/src/backend/rewrite/rewriteManip.c
index abad1bf7e4..f1f4212b5d 100644
--- a/src/backend/rewrite/rewriteManip.c
+++ b/src/backend/rewrite/rewriteManip.c
@@ -1205,7 +1205,7 @@ replace_rte_variables_mutator(Node *node,
  * If the expression tree contains a whole-row Var for the target RTE,
  * *found_whole_row is set to true.  In addition, if to_rowtype is
  * not InvalidOid, we replace the Var with a Var of that vartype, inserting
- * a ConvertRowTypeExpr to map back to the rowtype expected by the expression.
+ * a ConvertRowtypeExpr to map back to the rowtype expected by the expression.
  * (Therefore, to_rowtype had better be a child rowtype of the rowtype of the
  * RTE we're changing references to.)  Callers that don't provide to_rowtype
  * should report an error if *found_row_type is true; we don't do that here