correct path in cumulus README (#7001)

# Description

This PR fixes the file path in `cumulus/README.md` so the link to the
container documentation points to the correct location.

## Review Notes

The only change are the links in `cumulus/README.md` from
`./docs/contributor/container.md` to `../docs/contributor/container.md`.

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [x] My PR follows the [labeling requirements](

https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process
) of this project (at minimum one label for `T` required)
* External contributors: ask maintainers to put the right label on your
PR.
* [ ] I have made corresponding changes to the documentation (if
applicable)
* [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

Co-authored-by: Guillaume Thiolliere <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>