Fix documentation for 'model' parameter in CohereEmbeddings (#797)

Currently, the class parameter 'model_name' of the CohereEmbeddings
class is not supported, but 'model' is. The class documentation is
inconsistent with this, though, so I propose to either fix the
documentation (this PR right now) or fix the parameter.

It will create the following error:
```
ValidationError: 1 validation error for CohereEmbeddings
model_name
  extra fields not permitted (type=value_error.extra)
```

langchain/embeddings/cohere.py

@@ -18,7 +18,7 @@ class CohereEmbeddings(BaseModel, Embeddings):
         .. code-block:: python
 
             from langchain.embeddings import CohereEmbeddings
-            cohere = CohereEmbeddings(model_name="medium", cohere_api_key="my-api-key")
+            cohere = CohereEmbeddings(model="medium", cohere_api_key="my-api-key")
     """
 
     client: Any  #: :meta private: