Remove leftover reference to previously removed code example in Ch 16-03.

[apogeeoak]

When the Sharing a Mutex Between Multiple Threads section was updated in #1779 a reference to the now removed unrolled example was leaked in the following section. This pull request addresses that by also removing this dangling reference.

[apogeeoak]

The run lints check fails because even though the nightly toolchain is set as the default in the Install Rust step it is overridden by the 1.48 version that is set in the rust-toolchain file. The 1.48 version can be overridden by using rustup override set nightly in the main.yml workflow file.

[carols10cents]

Thanks, great catch!