doc: fix typo in Buffer.prototype.fill()

Refs: nodejs#17427
PR-URL: nodejs#17501
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/buffer.md

@@ -1276,7 +1276,7 @@ const buf = Buffer.allocUnsafe(5);
 console.log(buf.fill('a'));
 // Prints: <Buffer aa aa aa aa aa>
 console.log(buf.fill('aazz', 'hex'));
-// Throws a exception.
+// Throws an exception.
 console.log(buf.fill('zz', 'hex'));
 ```