Fix LinearTrend changepoints in plot_curve method

[gdeleva]

We corrected the formula for the change point time positions in the docstrings and aligned the vertical lines in the plot_curve function with the actual change point positions.

Description

Correct the formula of the position of the change points;
Align the vertical lines of plot_curve to the position of the change points.

Related Issue

• Closes Linear trend Changepoints Misaligned in Plot #1672
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1739.org.readthedocs.build/en/1739/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.59%. Comparing base (4eecb28) to head (7fb6781).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1739   +/-   ##
=======================================
  Coverage   91.59%   91.59%           
=======================================
  Files          60       60           
  Lines        6780     6780           
=======================================
  Hits         6210     6210           
  Misses        570      570           

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[williambdean]

Thanks for the PR @gdeleva!

[williambdean]

Thanks for the PR!