Update build-book.yaml to fix input workflow

[jukent]

From my branch action's error:

Run dawidd6/[email protected]
==> Repository: jukent/projectpythia.github.io
==> Artifact name: repo-zip
==> Local path: ./
==> Workflow name: input.trigger_workflow
==> Workflow conclusion: success
==> Allow forks: true
Error: Not Found - https://docs.github.com/rest/actions/workflow-runs#list-workflow-runs-for-a-workflow

I made one of 2 mistakes (or both). The action looks for a workflow called input.trigger_workflow, instead of looking for what was inputed. It should be inputs (plural) and potentially have curly brackets around it to indicate that it is a variable.

Related to ProjectPythia/projectpythia.github.io#319 and ProjectPythia/projectpythia.github.io#415