Fix gramitcal error in read_dir example

Author: Ali Raheem

src/libstd/fs.rs

@@ -2002,7 +2002,7 @@ pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// use std::{fs, io};
 ///
 /// fn main() -> io::Result<()> {
-///     // The order read_dir returns entries is not guaranteed. If reproducible
+///     // The order in which `read_dir` returns entries is not guaranteed. If reproducible
 ///     // ordering is required the entries should be explicitly sorted.
 ///     let mut entries = fs::read_dir(".")?
 ///         .map(|res| res.map(|e| e.path()))