Use TLSv1.3 with curl if specified at all

The curl option specified to use TLSv1.2 explicity while nowadays 1.3
is availalble and recommended.
Switch to specifying 1.3 instead of 1.2.

doc/src/installation/index.md

@@ -49,7 +49,7 @@ installation of `rustup` and then install `nightly` along with `clippy` or
 `miri`, first install `rustup` without a toolchain:
 
 ```console
-curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --default-toolchain none -y
+curl --proto '=https' --tlsv1.3 -sSf https://sh.rustup.rs | sh -s -- --default-toolchain none -y
 ```
 
 Next you can install `nightly` allowing `rustup` to downgrade until it finds