Update Dutch password_hint translation to correspond with validation rule

At the moment the translation says ```Minimaal 5 tekens``` which means your password should be at least 5 characters long. But a 5 character long password is not allowed by the validator. 

I think this was a translation error from the English one where it says ```Must be over 5 characters```. To make the Dutch translation correct the Dutch translation should be changed to ```Minimaal 6 tekens```.

```
    /**
     * Get a validator for an incoming registration request.
     *
     * @param  array $data
     * @return \Illuminate\Contracts\Validation\Validator
     */
    protected function validator(array $data)
    {
        return Validator::make($data, [
            'name' => 'required|max:255',
            'email' => 'required|email|max:255|unique:users',
            'password' => 'required|min:6',
        ]);
    }
```

resources/lang/nl/auth.php

@@ -27,7 +27,7 @@ return [
     'email' => 'Email',
     'password' => 'Wachtwoord',
     'password_confirm' => 'Wachtwoord Bevestigen',
-    'password_hint' => 'Minimaal 5 tekens',
+    'password_hint' => 'Minimaal 6 tekens',
     'forgot_password' => 'Wachtwoord vergeten?',
     'remember_me' => 'Mij onthouden',
     'ldap_email_hint' => 'Geef een email op waarmee je dit account wilt gebruiken.',