doc: use serial comma in buffer docs

Refs: nodejs/node#11321
Refs: nodejs/node#17384

PR-URL: nodejs/node#43048
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Akhil Marsonya <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>

Author: tniessen

doc/api/buffer.md

@@ -134,7 +134,7 @@ console.log(Buffer.from('fhqwhgads', 'utf16le'));
 ```
 
 Node.js buffers accept all case variations of encoding strings that they
-receive. For example, UTF-8 can be specified as `'utf8'`, `'UTF8'` or `'uTf8'`.
+receive. For example, UTF-8 can be specified as `'utf8'`, `'UTF8'`, or `'uTf8'`.
 
 The character encodings currently supported by Node.js are the following: