boards: arm: nucleo_f746zg: fix empty image

The en.high-perf_nucleo-144_mbed.jpg is an empty file.
This causes error when generating PDF documentation
as the tool cannot deal with empty image files.
So replace it with the image with same filename from
nucleo_f765zg, as the images are identical from
the online product description pages.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>