Fix struct reference in documentation

[yanorei32]

I tried to work with the smart handles version of this library.
But I got a warning from cargo doc.

warning: unresolved link to `File`
  --> src/filesystem/directory.rs:54:47
   |
54 |     /// Convert a raw file into a droppable [`File`]
   |                                               ^^^^ no item named `File` in scope
   |
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: `embedded-sdmmc` (lib doc) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

This PR fixes it.

[thejpster]

Damn. We should build the docs in the github actions.

Thank you for the fix!