Update the rustc badge to better reflect the supported version.
Wasmtime documentation says stable is the supported rustc version, and that's
what we test with CI, so the badge should reflect that.
Wasmtime doesn't even build with 1.37 any longer anyway.
<p>
<a href="https://github.com/bytecodealliance/wasmtime/actions?query=workflow%3ACI"><img src="https://github.com/bytecodealliance/wasmtime/workflows/CI/badge.svg" alt="build status" /></a>
<a href="https://bytecodealliance.zulipchat.com/#narrow/stream/217126-wasmtime"><img src="https://img.shields.io/badge/zulip-join_chat-brightgreen.svg" alt="zulip chat" /></a>
- <img src="https://img.shields.io/badge/rustc-1.37+-green.svg" alt="min rustc" />
+ <img src="https://img.shields.io/badge/rustc-stable+-green.svg" alt="supported rustc stable" />
<a href="https://docs.rs/wasmtime"><img src="https://docs.rs/wasmtime/badge.svg" alt="Documentation Status" /></a>
</p>
