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MINOR: [JS][Docs] Correct typo in README (#38658)


### Rationale for this change

A demo variable is named 'uft8' but should clearly be 'utf8'.

### What changes are included in this PR?

Two transpostions of two characters

### Are these changes tested?

N/A

### Are there any user-facing changes?

No

Authored-by: Benjamin Schmidt <bmschmidt@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>