docs: update docstring of to_json() method (#511)

from #494, 

this PR updated the docstring of ``to_json`` method in the credential so that it points user to ``from_authorized_user_info`` instead of the non-existent ``from_json``

Author: alvyjudy

packages/google-auth/google/oauth2/credentials.py

@@ -300,8 +300,9 @@ def to_json(self, strip=None):
                                    generated JSON.
 
         Returns:
-            str: A JSON representation of this instance, suitable to pass to
-                 from_json().
+            str: A JSON representation of this instance. When converted into
+            a dictionary, it can be passed to from_authorized_user_info()
+            to create a new credential instance.
         """
         prep = {
             "token": self.token,