Broken links in Before You Begin section of README.md

[davidbenncsiro]

The following links in the Before You Begin section of README.md are broken:

• https://github.com/Xilinx/Vitis-AI/blob/master/docs/learn/system_requirements.md
• https://github.com/Xilinx/Vitis-AI/tree/master/setup/mpsoc/VART#step2-setup-the-target
  • This one instead: https://github.com/Xilinx/Vitis-AI/tree/1.4/setup/mpsoc/VART ?