doc: fix blob.bytes() heading level

PR-URL: nodejs#60252
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: xty

doc/api/buffer.md

@@ -513,7 +513,7 @@ added:
 Returns a promise that fulfills with an {ArrayBuffer} containing a copy of
 the `Blob` data.
 
-#### `blob.bytes()`
+### `blob.bytes()`
 
 <!-- YAML
 added: