Fix connection config in README.md (#311)

Author: d52b8

README.md

@@ -51,7 +51,7 @@ return [
     'components' => [
         'mongodb' => [
             'class' => '\yii\mongodb\Connection',
-            'dsn' => 'mongodb://@localhost:27017/mydatabase',
+            'dsn' => 'mongodb://localhost:27017/mydatabase',
             'options' => [
                 "username" => "Username",
                 "password" => "Password"