doc: update manual checkout version

Author: manyinsects

README.md

@@ -57,8 +57,8 @@ When you get a permission denied message then you have to exclude the directory
 Basically: Clone the repository at the latest tag, run `cargo install`.
 
 ```bash
-# find out the latest version at https://github.com/rust-lang/rustlings/releases/latest (on edit 4.6.0)
-git clone -b 4.6.0 --depth 1 https://github.com/rust-lang/rustlings
+# find out the latest version at https://github.com/rust-lang/rustlings/releases/latest (on edit 4.7.1)
+git clone -b 4.7.1 --depth 1 https://github.com/rust-lang/rustlings
 cd rustlings
 cargo install --force --path .
 ```