Update README.md

Based on MDN documentation of `scrollIntoView` the options for `behavior` are `auto` and `smooth` not `instant`.

Updating documentation to reflect that for users who read this for insight into how to write a custom scroll function (for things like offset for example).

https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollIntoView#parameters

Author: EliRobinson

README.md

@@ -89,7 +89,7 @@ import { HashLink } from 'react-router-hash-link';
 
 <HashLink
   to="/path#hash"
-  scroll={(el) => el.scrollIntoView({ behavior: 'instant', block: 'end' })}
+  scroll={(el) => el.scrollIntoView({ behavior: 'auto', block: 'end' })}
 >
   Link to Hash Fragment
 </HashLink>;