Update README.md - remove unnecessary prefix
Update README.md - remove unnecessary prefix

Remove prefix because of this CLI message when running `yo generator-jhipster-db-helper`:
```
In the future, run yo jhipster-db-helper instead!

Error generator-jhipster-db-helper

You don't seem to have a generator with the name “jhipster-db-helper” installed.
```
bastienmichaux/generator-jhipster-db-helper,bastienmichaux/generator-jhipster-db-helper