doc: fix typo in dns.md

PR-URL: nodejs#17502
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/dns.md

@@ -514,7 +514,7 @@ will be present on the object:
 | `"SRV"` | Refer to [`dns.resolveSrv()`][] |
 | `"TXT"` | This type of record contains an array property called `entries` which refers to [`dns.resolveTxt()`][], eg. `{ entries: ['...'], type: 'TXT' }` |
 
-Here is a example of the `ret` object passed to the callback:
+Here is an example of the `ret` object passed to the callback:
 
 <!-- eslint-disable semi -->
 ```js