docs: fix typo ("a arbitrary" -> "an arbitrary") (tailwindlabs#18227)

## Summary
- Fixes a typo: "a arbitrary" → "an arbitrary" in a comment/description.

## Details
- This is a documentation-only change. No code logic is affected.

## Test Plan
- N/A (doc-only)

Co-authored-by: 中野 博文 <hirofumi0082@gmail.com>

Author: nakanoh

packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.test.ts

@@ -277,7 +277,7 @@ describe.each([['@theme'], ['@theme inline']])('%s', (theme) => {
   })
 })
 
-test('migrate a arbitrary property without spaces, to a theme value with spaces (canonicalization)', async () => {
+test('migrate an arbitrary property without spaces, to a theme value with spaces (canonicalization)', async () => {
   let candidate = 'font-[foo,bar,baz]'
   let expected = 'font-example'
   let input = css`