Fix broken links in csharp docs

[HomelessDinosaur]

From Elisse:

I'm pretty sure Rak had a PR for this, but I can't seem to find it... can anyone else?

These two pages:

https://nitric.io/docs/reference/csharp/v0/storage/bucket
https://nitric.io/docs/reference/csharp/v0/storage/bucket-file

Have links to:
file.signUrl()

that results in a 404.