Add ARGOCD_ENV_ prefix to AVP_SECRET variable in configuration documentation

[IvanVojtko]

Description

All environment variables should start with ARGOCD_ENV_, when configuring multitenancy using documentation, it doesn't work as documentation mentions only AVP_SECRET which doesn't exists, variable should be referenced as ARGOCD_ENV_AVP_SECRET.

Fixes: <! -- link to issue -->

Checklist

Please make sure that your PR fulfills the following requirements:

• Reviewed the guidelines for contributing to this repository
• The commit message follows the Conventional Commits Guidelines.
• Tests for the changes have been updated
• Are you adding dependencies? If so, please run go mod tidy -compat=1.21 to ensure only the minimum is pulled in.
• Docs have been added / updated
• Optional. My organization is added to USERS.md.

Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

Other information