doc: skip jb init when using Tanka

[daixiang0]

tk init would create file and make jb init return error:

[root config]# tk init
Directory structure set up! Remember to configure the API endpoint:
`tk env set environments/default --server=127.0.0.1:6443`
[root config]# tk env add environments/default
[root config]# jb init
jb: error: jsonnetfile.json already exists

Signed-off-by: Xiang Dai 764524258@qq.com

[sh0rez]

Hi, that line is indeed not required! LGTM!