docs: fix incorrect dict_id() reference

Fix documentation. The way to fetch the dict id from a dictionary is
`dict_data.dict_id()`. The function `zstanard.dictionary_id()` is not defined.

Closes #214.

Author: bisho

zstandard/backend_cffi.py

@@ -2635,7 +2635,7 @@ class ZstdCompressionDict(object):
 
     Dictionaries have unique integer IDs. You can retrieve this ID via:
 
-    >>> dict_id = zstandard.dictionary_id(dict_data)
+    >>> dict_id = dict_data.dict_id()
 
     You can obtain the raw data in the dict (useful for persisting and constructing
     a ``ZstdCompressionDict`` later) via ``as_bytes()``: