Remove redundant explicit link target in documentation

[striezel]

When the CI tries to generate the documentation, it fails with

error: redundant explicit link target
Error:    --> dotenv/src/lib.rs:218:51
    |
218 | /// Loads environment variables from [`io::Read`](std::io::Read).
    |                                       ----------  ^^^^^^^^^^^^^ explicit target is redundant
    |                                       |
    |                                       because label contains path that resolves to same destination
    |
    = note: when a link's destination is not specified,
            the label is used to resolve intra-doc links
    = note: `-D rustdoc::redundant-explicit-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::redundant_explicit_links)]`
help: remove explicit link target
    |
218 | /// Loads environment variables from [`io::Read`].
    |                                      ~~~~~~~~~~~~

error: redundant explicit link target

This can be seen in https://github.com/allan2/dotenvy/actions/runs/6670434445/job/18130274835, for example.

All instances of this redundant link target are fixed so that the documentation job should pass again.

[striezel]

@allan2: Together with the changes from #87 all CI jobs should be green.

[allan2]

Thanks! Appreciate your three PRs.