Fix fir_1x16_bit filter documentation

[fir_1x16_bit](https://github.com/xmos/lib_mic_array/blob/develop/lib_mic_array/api/mic_array/etc/fir_1x16_bit.h) 

It refers to the third parameter N_256, which is supposed to be the number of 256 coeff slices, that is not correct. The current form of the function supports only 1 slice and doesnt take in a third parameter.