README is wrong (for v1 only)

[4GUO]

The install command is for v1 wrongly:

go get github.com/casbin/xorm-adapter should be: go get github.com/casbin/xorm-adapter/v2

[casbin-bot]

@tangyang9464 @JalinWang @imp2002