Fix image URL for tree
Fix image URL for tree

The URL had been pointing to a GitHub blob view, which GitHub's markdown renderer seems to support by changing the image src into the URL of the raw content while adding a link to the blob, but [crates.io doesn't support this](https://crates.io/crates/sled) and so you got a broken image.
spacejam/sled,spacejam/sled,spacejam/sled