Fix span layer

[hanjinliu]

Fix legend of a spans layer

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.92%. Comparing base (cd7bcaf) to head (2add5fd).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
whitecanvas/layers/_primitive/spans.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   83.93%   83.92%   -0.01%     
==========================================
  Files         198      198              
  Lines       22639    22645       +6     
  Branches     2473     2475       +2     
==========================================
+ Hits        19001    19005       +4     
- Misses       2662     2663       +1     
- Partials      976      977       +1     

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