chore(cfnspec): Add coverage directory to npmignore (backport aws#20016…

…) (aws#20931)

This is an automatic backport of pull request aws#20016 done by [Mergify](https://mergify.com).


---


<details>
<summary>Mergify commands and options</summary>

<br />

More conditions and actions can be found in the [documentation](https://docs.mergify.com/).

You can also trigger Mergify actions by commenting on this pull request:

- `@Mergifyio refresh` will re-evaluate the rules
- `@Mergifyio rebase` will rebase this PR on its base branch
- `@Mergifyio update` will merge the base branch into this PR
- `@Mergifyio backport <destination>` will backport this PR on `<destination>` branch

Additionally, on Mergify [dashboard](https://dashboard.mergify.com/) you can:

- look at your merge queues
- generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com
</details>

packages/@aws-cdk/cfnspec/.npmignore

@@ -12,3 +12,4 @@ test/
 jest.config.js
 **/*.integ.snapshot
 **/*.integ.snapshot
+coverage/