docs: fix variable notation

Author: kenjis

user_guide_src/source/database/connecting.rst

@@ -32,7 +32,7 @@ Available Parameters
 
 **\\Config\\Database::connect($group = null, bool $getShared = true): BaseConnection**
 
-#. ``$group``: The database group name, a string that must match the config class' property name. Default value is ``$config->defaultGroup``.
+#. ``$group``: The database group name, a string that must match the config class' property name. Default value is ``Config\Database::$defaultGroup``.
 #. ``$getShared``: true/false (boolean). Whether to return the shared connection (see
    Connecting to Multiple Databases below).