Update parse.ts

Author: masx200

basic-calculator-iv/parse.ts

@@ -1,11 +1,11 @@
 import { Poly } from "./Poly.ts";
 /**  我们依然使用递归下降法来解决这个问题，文法如下：
 
-exp :=( exp "+"||"-" additive) || additive
+expression :=( expression "+"||"-" additive) || additive
 
 additive := (additive "*" factor) || additive
 
-factor := num || "("exp")" || variable(变量)
+factor := num || "("expression")" || variable(变量)
  */
 export function parse(tokens: (string | number)[]): Poly {
     let index = 0;