docs: fix environment variable names in Actions

[alvarogonzalez-packlink]

Description

Two of the examples in Actions docs are indicating to use DATAHUB_ACTIONS_DATAHUB_BASE_URL as a environment variable name instead of DATAHUB_ACTIONS_TEAMS_DATAHUB_BASE_URL and DATAHUB_ACTIONS_SLACK_DATAHUB_BASE_URL

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[anshbansal]

PRs go to https://github.com/datahub-project/datahub