Update string representation of yasa.Hypnogram

[raphaelvallat]

Closes #128

Let me know if the text looks good. Happy to make edits.

[codecov-commenter]

Codecov Report

Base: 92.56% // Head: 92.56% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (61c642a) compared to base (37058f2).
Patch coverage: 50.00% of modified lines in pull request are covered.

❗ Current head 61c642a differs from pull request most recent head 701f7f9. Consider uploading reports for the commit 701f7f9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   92.56%   92.56%           
=======================================
  Files          23       23           
  Lines        3119     3121    +2     
=======================================
+ Hits         2887     2889    +2     
  Misses        232      232           

Impacted Files	Coverage Δ
yasa/hypno.py	98.31% <50.00%> (-0.28%)	⬇️
yasa/detection.py	97.85% <0.00%> (+0.11%)	⬆️

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[remrama]

Approved 💃 with some thoughts to take or leave