docs: fix Chinese README translation - #1429

Summary

Closes #576.

Fixes two mistranslated passages in the Chinese README:

• Clarifies that callers must create the directory when destination is a function, while Multer creates it when destination is a string.
• Clarifies that req and file are the two inputs passed to the storage callbacks to help make the decision.

Verification

PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH npm run lint
PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH npm test
git diff --check

Results: lint passed, full test suite passed with 80 passing tests, and diff check passed.