README should not steer new users to overriding submit script template

The current example configuration code in the README steers new users in an unhelpful direction:

https://github.com/jupyterhub/batchspawner/blob/f4659678061dacf5837fb15ce340230dca18f3ca/README.md?plain=1#L84-L93

If copied, this sample overrides the template script provided with the class, losing features that are implemented with template logic. The sample code should be replaced with one that uses the provided configuration variables (`req_*` etc) in accordance with recommended practice.