docs: update how_to_import_and_export_data.md

[zhyass]

https://github.com/xelabs/go-mydumper/blob/master/README.md

[codecov]

Codecov Report

Merging #728 (c3398b8) into master (0430b4e) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #728   +/-   ##
=====================================
  Coverage      89%    89%           
=====================================
  Files         133    133           
  Lines        9478   9478           
=====================================
+ Hits         8467   8473    +6     
+ Misses        669    666    -3     
+ Partials      342    339    -3     

Impacted Files	Coverage Δ
src/backend/txn.go	87% <0%> (+<1%)	⬆️
src/backend/xacheck.go	81% <0%> (+1%)	⬆️
src/executor/engine/merge_join.go	96% <0%> (+1%)	⬆️

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