ipynb: simplify command to launch PCMark
ipynb: simplify command to launch PCMark

Use the monkeyrunner tool to launch PCMark. Also improves readability of the
instruction that starts the activity.

Signed-off-by: Michele Di Giorgio <99fd0b6f5a4e02a4022988691284da2a0299b046@arm.com>

ARM-software/lisa,credp/lisa,credp/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa,joelagnel/lisa,ARM-software/lisa,mdigiorgio/lisa,arnoldlu/lisa,arnoldlu/lisa,mdigiorgio/lisa,joelagnel/lisa,JaviMerino/lisa,bjackman/lisa,credp/lisa,ARM-software/lisa