Clarify slow sampling speed display unit in progressbar (s/draws → s/draw)#7797

[premshaw04]

Description

Related Issue

• Closes s/draws -> s/draw #7797
• Related to #

Checklist

• [*] Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [*] New feature / enhancement
• [*] Bug fix
• Documentation
• [*] Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7953.org.readthedocs.build/en/7953/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (a6e6fa8) to head (c752fb6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7953      +/-   ##
==========================================
+ Coverage   90.62%   91.47%   +0.85%     
==========================================
  Files         116      116              
  Lines       18947    18947              
==========================================
+ Hits        17170    17332     +162     
+ Misses       1777     1615     -162     

Files with missing lines	Coverage Δ
pymc/progress_bar.py	93.37% <100.00%> (ø)

... and 7 files with indirect coverage changes

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