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>

cmd/abigen/main.go

@@ -96,7 +96,7 @@ var (
 	}
 	aliasFlag = cli.StringFlag{
 		Name:  "alias",
-		Usage: "Comma separated aliases for function and event renaming, e.g. foo=bar",
+		Usage: "Comma separated aliases for function and event renaming, e.g. original1=alias1, original2=alias2",
 	}
 )