generalize readme

Author: RalfJung

README.md

@@ -61,8 +61,8 @@ rustup component add miri
 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, e.g. using
-`rustup install nightly-2019-03-28`.
+determine a nightly version that comes with Miri and install that using
+`rustup install nightly-YYYY-MM-DD`.
 
 Now you can run your project in Miri: