fix type in vocab config (#2977)

Author: joelgrus

allennlp/common/configuration.py

@@ -478,7 +478,7 @@ def configure(full_path: str = '') -> Config:
                    default_value=False,
                    comment="whether to extend the existing vocabulary (if you specified one)"),
         ConfigItem(name="min_count",
-                   annotation=int,
+                   annotation=Dict[str, int],
                    default_value=None,
                    comment="only include tokens that occur at least this many times"),
         ConfigItem(name="max_vocab_size",