Amend minimum Rust version.

1.56 now because of Rust 2021 edition dependency.

Author: nnethercote

README.md

@@ -13,7 +13,7 @@ To install from [crates.io](https://crates.io/):
 
 > `cargo install counts`
 
-This requires Rust 1.27 or later. The compiled binary will be put into
+This requires Rust 1.56 or later. The compiled binary will be put into
 `~/.cargo/bin/`.
 
 To update the installation: