Fix comments with "a expression"

src/backend/optimizer/path/equivclass.c

@@ -1001,7 +1001,7 @@ relation_can_be_sorted_early(PlannerInfo *root, RelOptInfo *rel,
 	}
 
 	/*
-	 * Try to find a expression computable from the reltarget.
+	 * Try to find an expression computable from the reltarget.
 	 */
 	em = find_computable_ec_member(root, ec, target->exprs, rel->relids,
 								   require_parallel_safe);

src/backend/parser/parse_expr.c

@@ -4251,7 +4251,7 @@ transformJsonFuncExprOutput(ParseState *pstate,	JsonFuncExpr *func,
 }
 
 /*
- * Coerce a expression in JSON DEFAULT behavior to the target output type.
+ * Coerce an expression in JSON DEFAULT behavior to the target output type.
  */
 static Node *
 coerceDefaultJsonExpr(ParseState *pstate, JsonExpr *jsexpr, Node *defexpr)