Add prepare callback usage to createSlice.md (#231)
Add prepare callback usage to createSlice.md (#231)

* Add prepare callback usage to createSlice.md

Added a description of the usage of a prepare callback to the createSlice API reference page. Also adapted the multiply case reducer in the example to demonstrate the usage.

* Reformatted changes in example

* Replace absolute link with a relative one

Co-Authored-By: Mark Erikson <f1b5a91d4d6ad523f2610114591c007e75d15084@isquaredsoftware.com>

reduxjs/redux-toolkit,reduxjs/redux-toolkit