message: fix demo CDN
diff: <!-- RELEASE (CDN)
  -->
  <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/dist/pptxgen.bundle.js"></script>
  - <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/demos/demo.js"></script>
  + <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/demos/common/demos.js"></script>
  <!--
  TODO: [only used during `-beta` dev cycles; update to below on release]
  <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@master/dist/pptxgen.bundle.js"></script>
  