Fix link to the HTML spec in PR template

Author: garyb

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 **Prerequisites**
 
-- [ ] Before opening a pull request, please check the HTML standard (https://dom.spec.whatwg.org/). If it doesn't appear in this spec, it may be present in the spec for one of the other `purescript-web` projects. Although MDN is a great resource, it is not a suitable reference for this project.
+- [ ] Before opening a pull request, please check the HTML standard (https://html.spec.whatwg.org/). If it doesn't appear in this spec, it may be present in the spec for one of the other `purescript-web` projects. Although MDN is a great resource, it is not a suitable reference for this project.
 
 
 **Description of the change**