docs: Fix import Client command (#268)

Co-authored-by: caitlinwheeless <caitlin@humansignal.com>

README.md

@@ -17,7 +17,7 @@
 > 
 > OR you can change your import statements as follows:
 > ```python
-> from label_studio_sdk._legacy import Client
+> from label_studio_sdk import Client
 > from label_studio_sdk.data_manager import Filters, Column, Operator, Type
 > from label_studio_sdk._legacy import Project
 > ```