DOC: use .svg replace .png image

[Zeroto521]

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[codecov]

Codecov Report

Merging #257 (d221f6e) into master (acd7066) will decrease coverage by 1.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
- Coverage   99.62%   97.95%   -1.67%     
==========================================
  Files          21       21              
  Lines         539      539              
  Branches       61       61              
==========================================
- Hits          537      528       -9     
- Misses          0        6       +6     
- Partials        2        5       +3     

Impacted Files	Coverage Δ
dtoolkit/accessor/_util.py	80.00% <0.00%> (-20.00%)	⬇️
dtoolkit/accessor/series.py	95.83% <0.00%> (-4.17%)	⬇️
dtoolkit/accessor/dataframe.py	93.90% <0.00%> (-3.66%)	⬇️

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