Remove redundant mut.

src/libstd/ffi/os_str.rs

@@ -191,7 +191,7 @@ pub fn push<T: AsRef<OsStr>>(&mut self, s: T) {
     /// ```
     /// use std::ffi::OsString;
     ///
-    /// let mut os_string = OsString::with_capacity(10);
+    /// let os_string = OsString::with_capacity(10);
     /// let capacity = os_string.capacity();
     ///
     /// // This push is done without reallocating