readme: fix srsRAN URL

[andrepuschmann]

Pull Request Details

This PR fixes the URL pointing to the srsRAN page in the readme.

Description

If people click on the srsRAN link they end up with a 404.

Checklist

Most of the below are not applicable.

• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project. See CODING.md.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes, and all previous tests pass.
• [ ] I have checked all compat numbers if they need updating (FPGA compat, MPM compat, noc_shell, specific RFNoC block, ...)

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