fixed bug with incorrect variable name in dataset_utils.py

Summary:
- ValueError was outputting `fn` instead of `func`
- Similar fix done in torchdata facebookexternal/torchdata#167

Reviewed By: ejguan

Differential Revision: D31149667

fbshipit-source-id: 2c1228287d513895f8359cb97935252f0087d738

Author: Nayef211

torchtext/data/datasets_utils.py

@@ -247,7 +247,7 @@ def decorator(func):
                 len(argspec.kwonlyargs) == 0 and
                 len(argspec.annotations) == 0
                 ):
-            raise ValueError("Internal Error: Given function {} did not adhere to standard signature.".format(fn))
+            raise ValueError("Internal Error: Given function {} did not adhere to standard signature.".format(func))
 
         @functools.wraps(func)
         def wrapper(root=os.path.expanduser('~/.torchtext/cache'), *args, **kwargs):