fix cache loading string instead of parsing json

app/corpora/cqi_over_socketio/cqi_corpora_corpus.py

@@ -54,7 +54,7 @@ def cqi_corpora_corpus_get_visualization_data(cqi_client: cqi.CQiClient, corpus_
     visualization_data_file_path = os.path.join(corpus.path, 'cwb', 'visualization_data.json')
     if os.path.exists(visualization_data_file_path):
         with open(visualization_data_file_path, 'r') as f:
-            payload = f.read()
+            payload = json.load(f)
         return {'code': 200, 'msg': 'OK', 'payload': payload}
     cqi_corpus = cqi_client.corpora.get(corpus_name)
     ##########################################################################