SOLR-14578: Update solrcloud-autoscaling-triggers.adoc and test

[MarcusSorealheis]

Description

fix action name in doc and test.

Solution

Change the execute action to be execute_plan

Tests

Fixes existing test

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the master branch.
• I have run ant precommit and the appropriate test suite.
• I have added tests for my changes.
• I have added documentation for the Ref Guide (for Solr changes only).