doc: fix typo in Buffer.from(string, [encoding])

PR-URL: #15013
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

doc/api/buffer.md

@@ -833,7 +833,7 @@ const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');
 console.log(buf2.toString());
 ```
 
-A `TypeError` will be thrown if `str` is not a string.
+A `TypeError` will be thrown if `string` is not a string.
 
 ### Class Method: Buffer.isBuffer(obj)
 <!-- YAML