Wrong example for mergeDeep method in documentation

### What happened
Wrong example for mergeDeep in docs. In live editor shown `merge` method which doesn't work as expected. If you run an example, output will be object with shallow merge and miscompare with expected output written in comments;
`merge(original, { x: { z: 456 }}) // { x: { y: 123, z: 456 }}`
https://facebook.github.io/immutable-js/docs/#/mergeDeep

### How to reproduce
Just run example in editor from this page https://facebook.github.io/immutable-js/docs/#/mergeDeep
And see an output.

Start at the linked Immutable.js documentation page for mergeDeep and inspect the live editor example alongside its expected output. Update the example so the demonstrated method and resulting nested object agree, then rerun the example to confirm the output matches the documentation.

Written by the indexing model from the issue text.