docs: remove CORS Credentials header

[lidel]

AFAIK Share App does not need this header.

See also:

• Potential problem with Access-Control-Allow-Credentials: https://github.com/ipfs/go-ipfs/issues/5745
• HTTP Headers Cleanup: https://github.com/ipfs/in-web-browsers/issues/132

[fsdiogo]

I copy-pasted this from Web UI without giving it much thought. Thanks @lidel!

[lidel]

No worries, I noticed that and removed from Web UI too: https://github.com/ipfs-shipyard/ipfs-webui/pull/886 :ok_hand: