fix link from #canvas -> #dialogcanvas in the Dialog api docs

[ninjamuffin99]

the internal link to the canvas section at the bottom of the Dialog page that I believe wants to link to the Dialog:canvas section, but my assumption is at some point the header link generation code may have changed

Pointing it towards #dialogcanvas should properly make that link at the bottom of the page there work

[dacap]

Thanks @ninjamuffin99!