docs: fix the link of "enhanceEndpoints"

[BeijiYang]

Issue: The link of "enhanceEndpoints" in rtkq document is wrong

In this page, the link of "enhanceEndpoints" is wrong.

Currently it's https://redux-toolkit.js.org/rtk-query/usage/code-splitting This page contains nothing about enhanceEndpoints.

It should be https://redux-toolkit.js.org/rtk-query/api/created-api/code-splitting#enhanceendpoints

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