cmd/abigen: clarify abigen alias flag usage (#21875)

* doc: clarify abigen alias flag usage

update the `abigen --alias` flag help info, give an example to make it more clear

related issue: https://github.com/ethereum/go-ethereum/issues/21846



* Update cmd/abigen/main.go

Co-authored-by: ligi <ligi@ligi.de>

Co-authored-by: Martin Holst Swende <martin@swende.se>
Co-authored-by: ligi <ligi@ligi.de>