improve placements type

[shaodahong]

Ref: ant-design/ant-design#19408

[vercel]

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[codecov]

Codecov Report

Merging #199 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #199   +/-   ##
=====================================
  Coverage      88%    88%           
=====================================
  Files           3      3           
  Lines          25     25           
  Branches       10     10           
=====================================
  Hits           22     22           
  Misses          3      3

Impacted Files	Coverage Δ
src/placements.tsx	100% <100%> (ø)	⬆️

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[zombieJ]

+ rc-tooltip@4.0.0-alpha.3