Fix 'client.get_config', too

Closes googleapis#3894.

runtimeconfig/google/cloud/runtimeconfig/config.py

@@ -186,7 +186,7 @@ def get_variable(self, variable_name, client=None):
 
           >>> from google.cloud import runtimeconfig
           >>> client = runtimeconfig.Client()
-          >>> config = client.get_config('my-config')
+          >>> config = client.config('my-config')
           >>> print(config.get_variable('variable-name'))
           <Variable: my-config, variable-name>
           >>> print(config.get_variable('does-not-exist'))