Fix typo in README.md

[JingchaoZhang]

correct typo

[codecov-commenter]

Codecov Report

Merging #209 (a4ac4d6) into master (ea32d45) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   80.07%   80.01%   -0.07%     
==========================================
  Files          54       54              
  Lines        4553     4553              
==========================================
- Hits         3646     3643       -3     
- Misses        907      910       +3     

Impacted Files	Coverage Δ
dpdata/__about__.py	0.00% <0.00%> (-100.00%)	⬇️
dpdata/__init__.py	76.47% <0.00%> (-11.77%)	⬇️

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