Reducer documentation needs updating

the Reducer documentation needs updating now that there is no Event trait

https://github.com/kellpossible/reactive-state/blob/03e62d7f29f0c34b2759e1df3c6b785b88e8f0af/src/reducer.rs#L99

Open src/reducer.rs at the referenced line and read the surrounding Reducer documentation. Check the current API to confirm that there is no Event trait, then update the documentation to match the current interface. Done means the Reducer documentation no longer describes the removed trait and remains accurate.

Written by the indexing model from the issue text.