docs: Update broken link to tutorial (#1112)

docs/src/tutorials/tutorial-4.md

@@ -58,4 +58,4 @@ try {
 
 It's that easy. :)
 
-To run the full example, go [here](https://github.com/project-serum/anchor/tree/master/examples/errors).
+To run the full example, go [here](https://github.com/project-serum/anchor/tree/master/examples/tutorial/basic-4).