Docs: Update organization limits screenshot to include configuration variables (#1073)

[ssmadhavan006]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [x] I have updated the documentation.

Reference to Existing Issue

Closes #1073.

Description of Changes

Updates the screenshot for the "Organization Limits" documentation page (docs/user/organization-limits.rst). The previous screenshot was outdated and did not show the "Configuration variables" field, which was added in a later release. The old external image link was also replaced with a relative path to incorporate the new image into the repository structure.

Screenshot