Switch zh tokenizer default pkuseg_model to spacy_ontonotes (explosio…

…n#12896)

So that users can use `copy_from_base_model` for other segmenters
without having to override an irrelevant `pkuseg_model` setting, switch
the default `pkuseg_model` to `spacy_ontonotes`.

spacy/lang/zh/__init__.py

@@ -31,7 +31,7 @@
 [initialize]
 
 [initialize.tokenizer]
-pkuseg_model = null
+pkuseg_model = "spacy_ontonotes"
 pkuseg_user_dict = "default"
 """