typo fix of doc of "nlpaug.augmenter.audio.crop"

[Owen864720655]

It seems that the illustration of the parameter coverage is not correct.

Old

For example, the audio duration is 60 seconds while zone and coverage are (0.2, 0.8) and 0.7 respectively. 42 seconds ((0.8-0.2)0.760) audio will be augmented.

New

For example, the audio duration is 60 seconds while zone and coverage are (0.2, 0.8) and 0.7 respectively. 25.2 seconds ((0.8-0.2)0.760) audio will be augmented.

[makcedward]

Will correct it in next release.