Casbin Auth Middleware's sample code has compilation error

[MasatoshiTada]

The code below has a compilation error:

e := echo.New()
e.Use(casbin_mw.Middleware(casbin.NewEnforcer("casbin_auth_model.conf", "casbin_auth_policy.csv")))

$ go build src/server.go 
# command-line-arguments
src/server.go:46:27: too many arguments in call to "github.com/labstack/echo-contrib/casbin".Middleware
        have (*"github.com/casbin/casbin".Enforcer, error)
        want (*"github.com/casbin/casbin".Enforcer)

We have to change this code like this:

e := echo.New()
enforcer, err := casbin.NewEnforcer("casbin_auth_model.conf", "casbin_auth_policy.csv")
e.Use(casbin_mw.Middleware(enforcer))

[MasatoshiTada]

I sent a PR. Please take a look!