Fix slack environment variable in in generative_chat example README

[bhatt-priyadutt]

Describe changes

I implemented/fixed the Bug to where the env variable specified in the docs should be use in the code as well.

#1834

Pre-requisites

Please ensure you have done the following:

• I have read the CONTRIBUTING.md document.
• If my change requires a change to docs, I have updated the documentation accordingly.
• If I have added an integration, I have updated the integrations table and the corresponding website section.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)

[bhatt-priyadutt]

@alex-zenml this can be merged as well

[strickvl]

Thanks for this catch, @bhatt-priyadutt !