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 <gui.thiolliere@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/README.md

@@ -4,7 +4,7 @@
 
 This repository contains both the Cumulus SDK and also specific chains implemented on top of this SDK.
 
-If you only want to run a **Polkadot Parachain Node**, check out our [container section](./docs/contributor/container.md).
+If you only want to run a **Polkadot Parachain Node**, check out our [container section](../docs/contributor/container.md).
 
 ## Cumulus SDK
 
@@ -34,7 +34,7 @@ A Polkadot [collator](https://wiki.polkadot.network/docs/en/learn-collator) for
 `polkadot-parachain` binary (previously called `polkadot-collator`).
 
 You may run `polkadot-parachain` locally after building it or using one of the container option described
-[here](./docs/contributor/container.md).
+[here](../docs/contributor/container.md).
 
 ### Relay Chain Interaction
 To operate a parachain node, a connection to the corresponding relay chain is necessary. This can be achieved in one of