Fix issue with --- still being required at foot of slide

[zolrath]

Pull request

Description

      // Files shouldn't have --- at the head or foot
      // It is added automatically here
      if (i + 1 < sourceUrls.length) {
        source += "---"
      }

Based off this comment in index.html each individual slide should not need to have a leading or trailing ---, but currently this does not work correctly.

The intro.md and slide1.md slides have a trailing --- at the foot of the file which allows the files to be split into individual pages.

The slide2.md file lacks the trailing --- and matching that style on the previous or future slides leads to files being merged into a single slide.

By adding source += "---\n---" instead, we can remove the footer --- from all slides and still have them display correctly.

Expectation

Please check all the relevant components;

• Code review && test

How to

• Pull this project and create a local webserver to serve its directory using the instructions listed in the README.md.
• Create a new slide and do not add a trailing --- to the footer.
• Add the new slide to the sourceUrls array in index.html.
• Observe the slides properly separated.

Alternatives

If the functionality listed in the comment in index.html isn't actually desired the comment can be changed.