fix book name by removing superfluous file extension

The workflow adds the `.pdf` extension to the name of the generated file, so this is duplicated

Author: AndrzejProchyra

.github/workflows/main.yml

@@ -15,4 +15,4 @@ jobs:
   build:
     uses: SquareBracketAssociates/BookBuilderDashboard/.github/workflows/main.yml@newVersionOfPillar
     with:
-      bookname: PharoByExample9.pdf
+      bookname: PharoByExample9