Update usrp_x4xx.dox for x440 MMPX

[benjamincitrin]

Pull Request Details

Description

Documents specify USRP X440 has MMCX ports on the front. This corrects the documentation, as the USRP X440 has MMPX ports.

Related Issue

Which devices/areas does this affect?

USRP X440

Testing Done

Checklist

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