[11.x] Fix withRouting docblock type

[santigarcor]

The docblock type definition for the withRoute is accepting string|null for api prefix, but It should only accept string

    /**
     * Register the routing services for the application.
     *
     * @param  \Closure|null  $using
     * @param  string|null  $web
     * @param  string|null  $api
     * @param  string|null  $commands
     * @param  string|null  $channels
     * @param  string|null  $pages
     * @param  string|null  $apiPrefix <--- it should be string only
     * @param  callable|null  $then
     * @return $this
     */
    public function withRouting(?Closure $using = null,
        ?string $web = null,
        ?string $api = null,
        ?string $commands = null,
        ?string $channels = null,
        ?string $pages = null,
        ?string $health = null,
        string $apiPrefix = 'api',
        ?callable $then = null)

It leads VSCode to show this kind of error:

[nunomaduro]

Thanks!