Fix an example warning

examples/russian.rs

@@ -29,11 +29,11 @@ fn main() {
         vec!["дек", "Декабрь"]
     ]);
 
-    let mut p = Parser::new(info);
+    let p = Parser::new(info);
 
     assert_eq!(
         p.parse("10 Сентябрь 2015 10:20", None, None, false, false, None, false, &HashMap::new())
             .unwrap().0,
         NaiveDate::from_ymd(2015, 9, 10).and_hms(10, 20, 0)
     );
-}
+}