notes.txt: Update notes.

doc/notes.txt

 
 ** add begining and ending measure numbers to 'seq' labels
 
-** Gain compensation fore mode 4.
+** Gain compensation for mode 4.
 
 ** Do IFFT using cos()^c + i * sin()^c - these bases are orthognal but cause harmonic distortion.
    To be efficient this might involve writing an FFT function.