tools/power/x86/amd_pstate_tracer: Fix python gnuplot package names

The prerequisites section listed non-existent packages
"phython-gnuplot" and "phython3-gnuplot", which may mislead users and
cause installation failures.

Update the names to the correct distribution package names
"python-gnuplot" and "python3-gnuplot", helping users avoid confusion
and saving time for those following the instructions.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>