Correct documentation URL for Value's Index impl.

The current id doesn't exist, so the link just goes to the top of the docs for
Value, rather than to the intended section covering `impl<I> Index<I> for
Value`.

src/value/index.rs

@@ -14,7 +14,7 @@ use core::ops;
 ///
 /// [`get`]: ../enum.Value.html#method.get
 /// [`get_mut`]: ../enum.Value.html#method.get_mut
-/// [square-bracket indexing operator]: ../enum.Value.html#impl-Index%3CI%3E
+/// [square-bracket indexing operator]: ../enum.Value.html#impl-Index%3CI%3E-for-Value
 ///
 /// This trait is sealed and cannot be implemented for types outside of
 /// `serde_json`.