docs: Fix bad import path in migration guide (#827)

You need the `google.cloud` namespace prefix.

Co-authored-by: Anthonios Partheniou <[email protected]>

Author: rwhogg

docs/migrating.rst

@@ -197,7 +197,7 @@ App Engine's legacy exceptions are no longer available, but `ndb` provides
 shims for most of them, which can be imported from the `ndb.exceptions`
 package, like this::
 
-    from ndb.exceptions import BadRequestError, BadArgumentError
+    from google.cloud.ndb.exceptions import BadRequestError, BadArgumentError
 
 Datastore API
 =============