Fix a typo in path.rs docs
The name of the variable used in the example is `path`, not `os_str`.
/// assert_eq!(path.to_string_lossy(), "foo.txt");
/// ```
///
- /// Had `os_str` contained invalid unicode, the `to_string_lossy` call might
+ /// Had `path` contained invalid unicode, the `to_string_lossy` call might
/// have returned `"fo�.txt"`.
#[stable(feature = "rust1", since = "1.0.0")]
pub fn to_string_lossy(&self) -> Cow<str> {