Use a more accurate model for symmetric semantic search

- The all-MiniLM-L6-v2 is more accurate
  - The exact previous model isn't benchmarked but based on the
    performance of the closest model to it. Seems like the new model
    maybe similar in speed and size

- On very preliminary evaluation of the model, the new model seems
  faster, with pretty decent results

src/utils/cli.py

@@ -79,7 +79,7 @@ default_config = {
     {
         'symmetric':
         {
-            'encoder': "sentence-transformers/paraphrase-MiniLM-L6-v2",
+            'encoder': "sentence-transformers/all-MiniLM-L6-v2",
             'cross-encoder': "cross-encoder/ms-marco-MiniLM-L-6-v2",
             'model_directory': None
         },