Change path to css file

It looks like this needs to be relative to the final html rather than the working directory when building(?)

.github/workflows/website.yml

@@ -129,7 +129,7 @@ jobs:
       - name: Build slides
         uses: docker://pandoc/core:latest
         with:
-          args: --to=revealjs --standalone -o slideshow/index.html --css=slideshow/mywhite.css  slideshow/slides.md
+          args: --to=revealjs --standalone -o slideshow/index.html --css=mywhite.css  slideshow/slides.md
 
       - name: Deploy slides
         uses: JamesIves/github-pages-deploy-action@v4