Fix a comment typo.

// Convert from UTF-16 to UTF-8.

std::string narrow(const wchar_t* utf16) {

    // Note: Do not call SystemErrorCodeToString() from narrow() because

    // SystemErrorCodeToString() calls narrows() and we don't want potential

    // SystemErrorCodeToString() calls narrow() and we don't want potential

    // infinite recursion.

    const int chars_required = WideCharToMultiByte(CP_UTF8, 0, utf16, -1, NULL,

                                                   0, NULL, NULL);