[Minor] Update the warning to follow the new conv_template file

[persistz]

Why are these changes needed?

In the commit #1008 , fastchat updated the model and conversation template, but ignored to sync the warning message in the model_adapter.py, which will cause a template cannot find error if user follows the command in the warning.

Related issue number (if applicable)

In several closed issues (#408 #422 ), the authors of fastchat quoted this warning message, this warning should be updated synchronously with the conv template.

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).

As this is only an updating of warning message, I think there is no related tests and docs to check here.