Fix stale requestFileHandles() call in Cross-Origin Storage docs (#27585)

The `instantiateWasm` example in
`site/source/docs/compiling/CrossOriginStorage.rst` called
`navigator.crossOriginStorage.requestFileHandles(cosHash)` with a bare
hash and then treated the result as a single `FileSystemFileHandle`
(`.then(handle => handle.getFile())`).

The API is `requestFileHandle()` (singular). The plural form was an
early draft that took an array of hashes and returned an array of
handles; it was [removed in favor of the singular
form](WICG/cross-origin-storage#61). The rest
of this document — and the actual implementation in `src/preamble.js` —
already use the singular form, so this example was the only stale call
site.

Docs-only, one line.

Author: tomayac

site/source/docs/compiling/CrossOriginStorage.rst

@@ -329,7 +329,7 @@ via a reference to that config object:
        // read the hash via the outer Module reference instead.
        const cosHash = Module['wasmHash'];
        if (cosHash?.value && globalThis.navigator?.crossOriginStorage) {
-         navigator.crossOriginStorage.requestFileHandles(cosHash)
+         navigator.crossOriginStorage.requestFileHandle(cosHash)
            .then(handle => handle.getFile())
            .then(f => f.arrayBuffer())
            .then(bytes => WebAssembly.instantiate(bytes, imports))