Tutorial #2 edits/comments

[rdgao]

• sine waves are incorrectly spelled as 'sin' in text
• says 3 parameters for butterworth filter but 4 are listed. Lower and upper bounds can be combined into the same bullet.
• time series plots are not the same size
• kernels are incorrectly spelled as 'kernal'
• arbitrary indexing magic numbers don't follow the (nice) convention of indices variables in other notebooks
• gaussian window is incorrectly referred to as a 'gaussian distribution'
• gaussian filter power needs to be normalized, otherwise filtered signal has a higher amplitude than the original
• link to [filtering tutorial in neurodsp] at the end (https://github.com/voytekresearch/neurodsp/blob/master/tutorials/1-Filtering.ipynb), which has more extensive coverage on frequency response of filters