Add MD extension to SmallSort challenge (#60)

The SmallSort challenge file is missing the `.md` extension, so our book renders an empty page (see [here](https://model-checking.github.io/verify-rust-std/challenges/0008-smallsort.html)). This PR adds the `.md` extension. I tested locally with `mdbook serve` to ensure it renders correctly. By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.
model-checking · Aug 20, 2024 · 62e241a · 62e241a
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diff --git a/doc/src/challenges/0008-smallsort → doc/src/challenges/0008-smallsort.md b/doc/src/challenges/0008-smallsort → doc/src/challenges/0008-smallsort.md