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.
• [x] 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
• [x] Documentation content changes
• [ ] Other (please describe)

Other information