[DOC] Glossary Mixup

[Ryan Hammonds]

@elkosik Brought to my attention that the definitions for rise and decay are flipped in the glossary:

 Rise (``time_rise``)
    The time between the current peak and the next trough.

Decay (``time_decay``)
    The time between the current peak and the last trough.

The code is correct though:

time_decay = df_samples['sample_next_trough'] - df_samples['sample_peak']
time_rise = df_samples['sample_peak'] - df_samples['sample_last_trough']

This is a reminder to fix this before the next release.