Update substitution_tag requirements

The notes state that type text is required for substition_tag, but it requires a type OpenTrackingSubstitutionTag

Author: jphilipsen05

sendgrid/helpers/mail/open_tracking.py

@@ -11,7 +11,7 @@ def __init__(self, enable=None, substitution_tag=None):
         :param enable: If open tracking is enabled.
         :type enable: boolean, optional
         :param substitution_tag: Tag in body to be replaced by tracking pixel.
-        :type substitution_tag: string, optional
+        :type substitution_tag: OpenTrackingSubstitionTag, optional
         """
         self._enable = None
         self._substitution_tag = None