Correction of the term "Quadratic Mean" for ensemble_averages in chapter 3: Random signals

[PrinzessinLea]

I suppose that the term "Quadratic Mean" is not the right term for the second raw moment. If I am right then you mean the Mean Square. In this case this term has to be changed in the reference card as well.

For a clear differentiation you may add a note, that the square root of a mean square is known as the root mean square (RMS or rms) that is also known as 'quadratic mean'.