MINOR: [JS][Docs] Correct typo in README (apache#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>

js/README.md

@@ -151,15 +151,15 @@ Strings can be encoded as UTF-8 or dictionary encoded UTF-8. Dictionary encoding
 ```js
 import { makeVector, vectorFromArray, Dictionary, Uint8, Utf8 } from "apache-arrow";
 
-const uft8Vector = vectorFromArray(['foo', 'bar', 'baz'], new Utf8);
+const utf8Vector = vectorFromArray(['foo', 'bar', 'baz'], new Utf8);
 
 const dictionaryVector1 = vectorFromArray(
     ['foo', 'bar', 'baz', 'foo', 'bar']
 );
 
 const dictionaryVector2 = makeVector({
     data: [0, 1, 2, 0, 1],  // indexes into the dictionary
-    dictionary: uft8Vector,
+    dictionary: utf8Vector,
     type: new Dictionary(new Utf8, new Uint8)
 });
 ```