Missing "title" attribute in "Go to permalink" modal dialogue

We are working our way through our XWiki instance with the WAVE plugin at the moment to increase accessibility, that is resolve all errors thrown by their standards.We found one very minor issue is a missing label or title attribute in a modal dialogue to copy a comments permalink. This PR is meant to fix this. It is my very first contribution to XWiki and I would be glad if there are any standards I missed, if you could advice me on where to look and what to fixWe already opened a PR on GitHub to resolve this issue.