Update create_metadata docstring (#80)

Author: alexandracota

labelbox/schema/data_row.py

@@ -37,7 +37,9 @@ def __init__(self, *args, **kwargs):
 
     def create_metadata(self, meta_type, meta_value):
         """ Creates an asset metadata for this DataRow.
-
+        
+        >>> datarow.create_metadata("TEXT", "This is a text message")
+        
         Args:
             meta_type (str): Asset metadata type, must be one of:
                 VIDEO, IMAGE, TEXT.