typo

modify `mull` to `null` in line 474 annotation.

examples/run_squad.py

@@ -471,7 +471,7 @@ def write_predictions(all_examples, all_features, all_results, n_best_size,
         prelim_predictions = []
         # keep track of the minimum score of null start+end of position 0
         score_null = 1000000  # large and positive
-        min_null_feature_index = 0  # the paragraph slice with min mull score
+        min_null_feature_index = 0  # the paragraph slice with min null score
         null_start_logit = 0  # the start logit at the slice with min null score
         null_end_logit = 0  # the end logit at the slice with min null score
         for (feature_index, feature) in enumerate(features):