Fix docstring formatting for clippy.

Fixes:

```
error: doc list item missing indentation
   --> src/commands/decompress.rs:237:5
    |
237 | /// Note: This functions assumes that `output_dir` exists
    |     ^
    |
    = help: if this is supposed to be its own paragraph, add a blank line
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_lazy_continuation
    = note: `-D clippy::doc-lazy-continuation` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::doc_lazy_continuation)]`
help: indent this line
    |
237 | ///   Note: This functions assumes that `output_dir` exists
    |     ++
```

src/commands/decompress.rs

@@ -234,6 +234,7 @@ pub fn decompress_file(
 /// - If the archive contains only one file, it will be extracted to the `output_dir`
 /// - If the archive contains multiple files, it will be extracted to a subdirectory of the
 ///   output_dir named after the archive (given by `output_file_path`)
+///
 /// Note: This functions assumes that `output_dir` exists
 fn smart_unpack(
     unpack_fn: impl FnOnce(&Path) -> crate::Result<usize>,