doc: fix wrong offset of example in Buffer.copyBytesFrom

Actual output is different from expected output because
wrong offset is used. Plus, removed unused importing `Buffer`.

Author: deokjinkim

doc/api/buffer.md

@@ -581,7 +581,7 @@ contained by the `Blob` is copied only when the `arrayBuffer()` or `text()`
 methods are called.
 
 ```mjs
-import { Blob, Buffer } from 'node:buffer';
+import { Blob } from 'node:buffer';
 import { setTimeout as delay } from 'node:timers/promises';
 
 const blob = new Blob(['hello there']);
@@ -608,7 +608,7 @@ blob.text().then(console.log);
 ```
 
 ```cjs
-const { Blob, Buffer } = require('node:buffer');
+const { Blob } = require('node:buffer');
 const { setTimeout: delay } = require('node:timers/promises');
 
 const blob = new Blob(['hello there']);
@@ -1075,7 +1075,7 @@ Copies the underlying memory of `view` into a new `Buffer`.
 
 ```js
 const u16 = new Uint16Array([0, 0xffff]);
-const buf = Buffer.copyBytesFrom(u16, 0, 1);
+const buf = Buffer.copyBytesFrom(u16, 1, 1);
 u16[1] = 0;
 console.log(buf.length); // 2
 console.log(buf[0]); // 255