chore: make the note on documentation PRs a comment (#1437)

Update PULL_REQUEST_TEMPLATE.md

Author: teemingc

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,4 @@
-### A note on documentation PRs
-
-If this is a documentation PR (i.e. changing content within `apps/svelte.dev/content/docs`), then this is the wrong repository to make those changes. The content in this folder is synced from other repositories. Therefore, these changes should be made in their respective repositories (at https://github.com/sveltejs/svelte or https://github.com/sveltejs/kit, or example).
+<!-- If this is a documentation PR (i.e. changing content within `apps/svelte.dev/content/docs`), then this is the wrong repository to make those changes. The content in this folder is synced from other repositories. Therefore, these changes should be made in their respective repositories (at https://github.com/sveltejs/svelte or https://github.com/sveltejs/kit, or example). -->
 
 ### Before submitting the PR, please make sure you do the following