[Doc] Updating documentation: Running the test multiple times

[yannicks1]

[Doc] Updating documentation: Running the test multiple times

This PR improves the section 'Running the test multiple times' in the readme and documentation. The current description is confusing and can be understood wrong:

import pyRAPL

  pyRAPL.setup()
	
  @pyRAPL.measureit(number=100)
  def foo():
      # Instructions to be evaluated.

  for _ in range(100):
      foo()

This code executes 100 experiments with 100 runs each, and does not only average over 100 runs as described.

In order to make things more clear, this commit changes the number of experiments to 20 and updates the description by distinguishing between runs and experiments.

[yannicks1]

Could anyone review and merge, @altor @chakib-belgaid @rouvoy @iivanoo ,thanks!