Link to markdown header in different file does not navigate correctly from notebook markdown cell

[amunger]

while testing #147731

1. create a markdown file with a header # header
2. create a notebook with a markdown cell and link to that header [link](document.md#header)
3. click the link in the rendered markdown

result:
vscode attempts to navigate to a file named document.md#header

note that both auto-complete and find references for the header work correctly for that link.