community: Fix the stop sequence key name for Mistral in Bedrock (#20709)
Fixing the wrong stop sequence key name that causes an error on AWS
Bedrock.
You can check the MistralAI bedrock parameters
[here](https://docs.aws.amazon.com/bedrock/latest/userguide/model-parameters-mistral.html)
This change fixes this
[issue](https://github.com/langchain-ai/langchain/issues/20095)