docs(nix/buildkite): escape "release" in warning r/6226

When using the buildkite tooling with `postBuild` parameter set, the
following error message is shown:

```
This replaces the previous boolean postBuild API in extra
step definitions. Please remove the postBuild parameter from
this step and instead set phase = release;.
```

However, `release` is no symbol, this should be a string.

Update the message to reflect that.

Change-Id: I341c81d862204db06946512428910cada28481ca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8690
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

0 files changed, 0 insertions, 0 deletions