SplitLab fix comment

[connortsui20]

    /// If the delimiter is the empty string, returns the next character (as a string) to avoid
    /// infinitely looping.

This is no longer true!

• Update the handout (splitlab)
• Update the refsol (splitlab_ref)
• Update the handout on the website by (in order):
  • copying and pasting the docs folder into there
  • re-zip the handout (make sure to delete any unnecessary files like Cargo.lock and everything in target/, including docs/ after you copy it over)