Docs: Fix Brockmann link in overview.rst

[fneu]

I noticed a markdown-formatted link in overview.rst.
It’s a minor thing, but it stood out when I was glancing through the docs.

Checklist:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/source/*
• [ ] Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[pont-us]

Somehow we overlooked this for a long time. Many thanks for the fix @fneu , and sorry about the delay!