Update Timeline Template README.md (#95814) (#100237)

## Summary

`xpack.security_solution.signalsIndex` is listed to be added to `kibana.dev.yml`, however that generates:
```
FATAL  Error: Unknown configuration key(s): "xpack.security_solution.signalsIndex". \
Check for spelling errors and ensure that expected plugins are installed.
```
I think this should be `xpack.securitySolution.signalsIndex`

This is for the steps to create a timeline template.

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html) was added for features that require explanation or tutorials

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

Co-authored-by: Andrew Pease <7442091+peasead@users.noreply.github.com>

x-pack/plugins/security_solution/server/lib/detection_engine/README.md

@@ -35,7 +35,7 @@ source ~/.zshrc
 Open your `kibana.dev.yml` file and add these lines:
 
 ```sh
-xpack.security_solution.signalsIndex: .siem-signals-${your user id}
+xpack.securitySolution.signalsIndex: .siem-signals-${your user id}
 ```
 
 Restart Kibana and ensure that you are using `--no-base-path` as changing the base path is a feature but will