Fix typo in toctree (#14704)

Author: mishig25

docs/source/_toctree.yml

@@ -167,7 +167,7 @@
     - local: model_doc/flaubert
       title: FlauBERT
     - local: model_doc/fnet
-      title: FlauBERT
+      title: FNet
     - local: model_doc/fsmt
       title: FSMT
     - local: model_doc/funnel