Tiny fix to addHotWord doc string parameters

As the parameter for boost was actually written as "word" in the doc string, it was replacing the previous type for word with the type intended for boost and not showing any type for boost, thus messing up what displayed on https://deepspeech.readthedocs.io/en/master/Python-API.html

native_client/python/__init__.py

@@ -102,8 +102,8 @@ class Model(object):
         :param word: the hot-word
         :type word: str
 
-        :param word: the boost
-        :type word: float
+        :param boost: the boost
+        :type boost: float
 
         :throws: RuntimeError on error
         """