Update doctest in comments.md

For a user following the path of reading Chapter 5: Syntax & Symantics
prior to Chapter 4: Learn Rust, this will be the first time they have
encountered executable tests inside documentation comments.

The test will fail because the `add_one` function is not defined in
the context of the doctest. This might not be the optimal place to
introduce and explain the `/// #` notation but I think it is important
that this snippet pass as a test when `rustdoc --test` is run against
it if it is going to be shown.

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