Update issue number of shrink_to methods to point the tracking issue

src/liballoc/vec.rs

@@ -613,7 +613,7 @@ impl<T> Vec<T> {
     /// vec.shrink_to(0);
     /// assert!(vec.capacity() >= 3);
     /// ```
-    #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
+    #[unstable(feature = "shrink_to", reason = "new API", issue="56431")]
     pub fn shrink_to(&mut self, min_capacity: usize) {
         self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
     }