fix link to iframe attribute on mdn

Author: nnmrts

docs/rules/iframe-missing-sandbox.md

@@ -4,7 +4,7 @@
 
 The sandbox attribute enables an extra set of restrictions for the content in the iframe. Using sandbox attribute is considered a good security practice.
 
-See <https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#attr-sandbox>
+See <https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#sandbox>
 
 ## Rule Details