fix(python): Fix plotting f-strings and docstrings

[hamdanal]

• Add missing f prefixes for f-strings
• Add missing line in docstring of DataFrame.plot.line
• Fix typo in docstring of Series.plot.line examples section

Closes #18475

[MarcoGorelli]

thanks @hamdanal !

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (dbd2c5f) to head (a94eb93).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/series/plotting.py	0.00%	2 Missing ⚠️
py-polars/polars/dataframe/plotting.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20399   +/-   ##
=======================================
  Coverage   78.95%   78.95%           
=======================================
  Files        1561     1561           
  Lines      219881   219881           
  Branches     2471     2471           
=======================================
  Hits       173614   173614           
  Misses      45699    45699           
  Partials      568      568           

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