[model_cards] Dataset ids are case-sensitive

cc @lhoestq @thomwolf

Also cc'ing model author @nreimers => Model pages now properly link to the dataset pages (and in the future, eval results, etc.)

model_cards/sentence-transformers/bert-base-nli-cls-token/README.md

@@ -4,8 +4,8 @@ tags:
 - exbert
 license: apache-2.0
 datasets:
-- SNLI
-- MultiNLI
+- snli
+- multi_nli
 ---
 
 # BERT base model (uncased) for Sentence Embeddings

model_cards/sentence-transformers/bert-base-nli-max-tokens/README.md

@@ -4,8 +4,8 @@ tags:
 - exbert
 license: apache-2.0
 datasets:
-- SNLI
-- MultiNLI
+- snli
+- multi_nli
 ---
 
 # BERT base model (uncased) for Sentence Embeddings

model_cards/sentence-transformers/bert-base-nli-mean-tokens/README.md

@@ -4,8 +4,8 @@ tags:
 - exbert
 license: apache-2.0
 datasets:
-- SNLI
-- MultiNLI
+- snli
+- multi_nli
 ---
 
 # BERT base model (uncased) for Sentence Embeddings