doc: fix ES module code blocks rendering in modules.md

The two consecutive `mjs` code blocks in the "Loading ECMAScript
modules using require()" section were being incorrectly grouped
into a CJS/ESM toggle tab by doc-kit's highlighter. Change the
language tag from `mjs` to `js` so they render as independent
code blocks.

Author: okuryu

doc/api/modules.md

@@ -219,12 +219,12 @@ directly.
 
 With the following ES Modules:
 
-```mjs
+```js
 // distance.mjs
 export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }
 ```
 
-```mjs
+```js
 // point.mjs
 export default class Point {
   constructor(x, y) { this.x = x; this.y = y; }