Updated README to reflect deprecation of rustup install

Author: Markus Pettersson

README.md

@@ -61,7 +61,7 @@ If `rustup` says the `miri` component is unavailable, that's because not all
 nightly releases come with all tools. Check out
 [this website](https://rust-lang.github.io/rustup-components-history) to
 determine a nightly version that comes with Miri and install that using
-`rustup install nightly-YYYY-MM-DD`.
+`rustup toolchain install nightly-YYYY-MM-DD`.
 
 Now you can run your project in Miri: