update docs index

docs/20_Checks/_index.md

@@ -21,6 +21,7 @@ There is one include script used by all checks:
 * [check_disk-io](check_disk-io.md)
 * [check_dns_responsetime](check_dns_responsetime.md)
 * [check_docker_info](check_docker_info.md)
+* [check_docker_stats](check_docker_stats.md)
 * [check_eol](check_eol.md)
 * [check_fs_errors](check_fs_errors.md)
 * [check_fs_writable](check_fs_writable.md)