docs: fix simple typo, signle -> single (#291)

There is a small typo in neuralcoref/train/document.py.

Should read `single` rather than `signle`.

Author: timgates42

neuralcoref/train/document.py

@@ -845,7 +845,7 @@ def set_mentions_features(self):
             mention.words_embeddings_ = words_embeddings_
 
     def get_single_mention_features(self, mention):
-        """ Features for anaphoricity test (signle mention features + genre if conll)"""
+        """ Features for anaphoricity test (single mention features + genre if conll)"""
         features_ = mention.features_
         features_["DocGenre"] = self.genre_
         return (features_, np.concatenate([mention.features, self.genre], axis=0))