Correct instruction to rebuild docs in CI workflow

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
         bash build.sh
         if [ -n "$(git status --porcelain)" ]; then
           echo "Documentation not up to date."
-          echo "Make sure to run 'build.sh' in the documentation/ folder"
+          echo "Make sure to run 'build.sh' in the book/ folder"
           exit 1
         fi