Fix metadata error

Should fix error:

YAML Metadata Error: "datasets[2]" with value "MS Marco" is not valid. It should not contain any whitespace. If possible, use a dataset id from the huggingface Hub.

README.md

@@ -9,7 +9,7 @@ license: apache-2.0
 datasets:
 - s2orc
 - flax-sentence-embeddings/stackexchange_xml
-- MS Marco
+- ms_marco
 - gooaq
 - yahoo_answers_topics
 - code_search_net