message: update sampler can use python 3 comment
diff: # run openocd (0.9.0) with :
  # $ openocd -f interface/stlink-v2.cfg -f target/stm32f4x.cfg &> /dev/null &
  # then run
  -# $ python2 sampler.py path_to_myelf_with_symbols
  +# $ python sampler.py path_to_myelf_with_symbols
  # ctrl-c to stop sampling.
  # To terminate the openocd session, enter command "fg" then do ctrl-c.
  