docs: correct docstring for the fns= parameter in the fmt() method

[rich-iannone]

This pull request makes a small change to the docstrings of the .fmt() method. The change corrects the text relating to the fns= parameter, clarifying that it only accepts a single formatting function (i.e., doesn't accept a list of functions).

This is related to an earlier PR (#327), which will be closed in favor of fixing the misleading documentation here (i.e., not modifying the code to fit the incorrect statement).

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.31%. Comparing base (a418234) to head (279c102).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
great_tables/_formats.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #818      +/-   ##
==========================================
- Coverage   92.32%   92.31%   -0.02%     
==========================================
  Files          49       49              
  Lines        6023     6026       +3     
==========================================
+ Hits         5561     5563       +2     
- Misses        462      463       +1     

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[machow]

LGTM thanks!