doc: fix wrong output of example in util

In `mime.type` of util, `application/javascript` is actual output,
but described as `application/javascript/javascript`.

PR-URL: #45825
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: deokjinkim

doc/api/util.md

@@ -1103,7 +1103,7 @@ myMIME.type = 'application';
 console.log(myMIME.type);
 // Prints: application
 console.log(String(myMIME));
-// Prints: application/javascript/javascript
+// Prints: application/javascript
 ```
 
 #### `mime.subtype`