Fix un-prefixed f-string

see https://github.com/huggingface/transformers/pull/4367#discussion_r426356693

Hat/tip @girishponkiya

examples/distillation/scripts/extract_distilbert.py

@@ -82,8 +82,8 @@ if __name__ == "__main__":
     compressed_sd["vocab_projector.bias"] = state_dict["cls.predictions.bias"]
     if args.vocab_transform:
         for w in ["weight", "bias"]:
-            compressed_sd[f"vocab_transform.{w}"] = state_dict["cls.predictions.transform.dense.{w}"]
-            compressed_sd[f"vocab_layer_norm.{w}"] = state_dict["cls.predictions.transform.LayerNorm.{w}"]
+            compressed_sd[f"vocab_transform.{w}"] = state_dict[f"cls.predictions.transform.dense.{w}"]
+            compressed_sd[f"vocab_layer_norm.{w}"] = state_dict[f"cls.predictions.transform.LayerNorm.{w}"]
 
     print(f"N layers selected for distillation: {std_idx}")
     print(f"Number of params transfered for distillation: {len(compressed_sd.keys())}")