Switched non-running `main` from `ignore` to `no_run`.

Fixed a bug that was uncovered by doing so (a missing `&mut`).

(Of course it is still the responsibility of the documenter to ensure
that the signatures of `fn render` are kept in sync across the blocks.
I do not really have much of a suggestion of a better way to handle
that problem, at least not one that does not involve more
sophisticated documentation generation tools.)