fskmodem/example: fixing typo in help with bits/symbol
fskmodem/example: fixing typo in help with bits/symbol

wangning223/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,JayKickliter/liquid-dsp,wangning223/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,JayKickliter/liquid-dsp,jgaeddert/liquid-dsp,JayKickliter/liquid-dsp,JayKickliter/liquid-dsp,JayKickliter/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp