Fix cap/floor bug in ProphetModel

[Mr-Geekman]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Look #841.

Closing issues

Closes #841.

[github-actions]

🚀 Deployed on https://deploy-preview-842--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #842 (e1525e2) into master (1565b18) will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master     #842   +/-   ##
=======================================
  Coverage   49.34%   49.34%           
=======================================
  Files         130      130           
  Lines        7409     7411    +2     
=======================================
+ Hits         3656     3657    +1     
- Misses       3753     3754    +1     

Impacted Files	Coverage Δ
etna/models/prophet.py	90.69% <33.33%> (-0.97%)	⬇️

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