recipes-apps: aie-matrix-multiplication: Update README
recipes-apps: aie-matrix-multiplication: Update README

* Reword the demo description
* Remove the timing information
* Remove the obsolete known issue

Signed-off-by: Wendy Liang <3b5b00198ad7f132308f5f249afae9e1b816a404@xilinx.com>
Signed-off-by: Manjukumar Matha <4c64a17569570cb9f1048973bce031c402b55312@xilinx.com>

Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux