Rename 2019/karns/demo.sh try.sh

It was already documented as such (in the thanks file) but it was still
the old name (demo.sh). The README.md had demo.sh still but this has
been updated as it's all for the try section.

2019/karns/README.md

@@ -26,7 +26,7 @@ The last two can be done in a sequence, first showing the respective files at
 the terminal, like:
 
 ```sh
-./demo.sh
+./try.sh
 ```