[Bug] example code in docs is wrong

[Frederic-Zhou]

In: https://casbin.org/docs/en/model-storage#load-model-from-string

// Load the policy rules from the .CSV file adapter.
// Replace it with your adapter to avoid files.
a := fileadapter.NewAdapter("examples/rbac_policy.csv")

// Create the enforcer.
e := casbin.NewEnforcer(m, a)

This is wrong:

a := model.NewModelFromString("examples/rbac_policy.csv")

and:

e := casbin.NewEnforcer(m, a)

[casbin-bot]

@closetool @tangyang9464

[hsluoyz]

@Frederic-Zhou plz change title and text to English, thanks!

[tangyang9464]

fixed. See casbin-website#269

[casbin-bot]

@ErikQQY