fix a typo

fastNLP/core/dataset.py

@@ -53,7 +53,7 @@ r"""
         from fastNLP import DataSet
         from fastNLP import Instance
         instances = []
-        winstances.append(Instance(sentence="This is the first instance",
+        instances.append(Instance(sentence="This is the first instance",
                             ords=['this', 'is', 'the', 'first', 'instance', '.'],
                             seq_len=6))
         instances.append(Instance(sentence="Second instance .",