Update deploy-book.yml to manually trigger the workflow for testing

.github/workflows/deploy-book.yml

@@ -1,7 +1,5 @@
 name: deploy-book
 
-name: deploy-book
-
 # Run this when the master or main branch changes
 on:
   push:
@@ -13,6 +11,9 @@ on:
     #
     # paths:
     # - some-subfolder/**
+    workflow_dispatch: # manually trigger the workflow for testing.
+      branches:
+      - main
 
 # This job installs dependencies, builds the book, and pushes it to `gh-pages`
 jobs: