chore: improve registerImage comment

[shaodahong]

Related #62 (comment)

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/react-component/image/d834tuher
✅ Preview: https://image-git-improve-comment.react-component.vercel.app

[codecov]

Codecov Report

Merging #63 (dc79bfb) into master (d0949af) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #63   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          230       230           
  Branches        63        63           
=========================================
  Hits           230       230           

Impacted Files	Coverage Δ
src/PreviewGroup.tsx	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d0949af...dc79bfb. Read the comment docs.