Fix punctuation.

src/libstd/ffi/os_str.rs

@@ -616,7 +616,7 @@ pub fn is_empty(&self) -> bool {
     /// Note that this does **not** return the number of bytes in the string in
     /// OS string form.
     ///
-    /// The length returned is that of the underlying storage used by `OsStr`;
+    /// The length returned is that of the underlying storage used by `OsStr`.
     /// As discussed in the [`OsString`] introduction, [`OsString`] and `OsStr`
     /// store strings in a form best suited for cheap inter-conversion between
     /// native-platform and Rust string forms, which may differ significantly