Fix clippy::doc_lazy_continuation warning

```
error: doc list item missing indentation
   --> src/lib.rs:174:5
    |
174 | /// does not already exist and it could not be created otherwise. The specific
    |     ^
    |
    = 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
    |
174 | ///   does not already exist and it could not be created otherwise. The specific
    |     ++

error: doc list item missing indentation
   --> src/lib.rs:175:5
    |
175 | /// error conditions for when a directory is being created (after it is
    |     ^
    |
    = 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
help: indent this line
    |
175 | ///   error conditions for when a directory is being created (after it is
    |     ++

error: doc list item missing indentation
   --> src/lib.rs:176:5
    |
176 | /// determined to not exist) are outlined by [`fs::create_dir`].
    |     ^
    |
    = 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
help: indent this line
    |
176 | ///   determined to not exist) are outlined by [`fs::create_dir`].
    |     ++
```

Author: taiki-e

src/lib.rs

@@ -171,9 +171,9 @@ pub async fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// limited to just these cases:
 ///
 /// * If any directory in the path specified by `path`
-/// does not already exist and it could not be created otherwise. The specific
-/// error conditions for when a directory is being created (after it is
-/// determined to not exist) are outlined by [`fs::create_dir`].
+///   does not already exist and it could not be created otherwise. The specific
+///   error conditions for when a directory is being created (after it is
+///   determined to not exist) are outlined by [`fs::create_dir`].
 ///
 /// Notable exception is made for situations where any of the directories
 /// specified in the `path` could not be created as it was being created concurrently.