'Show.sh' has lost .sh extension

[microbit-rosslowe]

Commit 99b8250 moved the show.sh script to the /bin/ directory, however the .sh extension was dropped. As a result, the readme is inaccurate:

Assuming you have Python 3 installed you can serve the editor like this:
$ ./bin/show.sh

The file should either be renamed to show.sh, or the readme changed.