docs: fix Intel GPU Usage in examples/README.md

[obaibula]

The example for Intel GPU usage parses the Render/3D value from the 8th field of intel_gpu_top -l output. Since this was introduced, the Render/3D field has shifted to the 9th position, breaking the example:

oleh@gentoooleh ~ $ sudo intel_gpu_top -l -n 1
 Freq MHz      IRQ RC6     Power W     IMC MiB/s             RCS             BCS             VCS            VECS 
 req  act       /s   %   gpu   pkg     rd     wr       %  se  wa       %  se  wa       %  se  wa       %  se  wa 
 388  388      582  50  0.95 36.54   3436   2309   42.43   0   0    0.00   0   0    0.00   0   0    0.00   0   0 

To prevent future field position shifts, I propose to utilize JSON output from intel_gpu_top -J and extract the Render/3D value by key with jq: [1].engines["Render/3D"].busy.

Note: the first sample in the JSON array always contains a high Render/3D spike, so the second sample ([1]) is used instead to get an accurate numbers. I also added this note to the file.

[ammgws]

Has the -J flag always been there?

[obaibula]

To be precise, the -J flag was added more than 7 years ago. Here is the commit: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/47e91cc220962fef2ad5afd1ce39fea1cec52f95

The -l flag used before my changes was added in the same commit as the -J flag.

[ammgws]

Thanks!