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.
# sled likes eating data! it's pre-alpha
<p>
- <img src="https://github.com/spacejam/rsdb/blob/master/art/tree_face.png" width="20%" height="auto" />
+ <img src="https://raw.githubusercontent.com/spacejam/sled/master/art/tree_face.png" width="20%" height="auto" />
</p>
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