Scintilla text editor can't find Ruby file (another really basic question)

[paul.denlinger]

I have installed Ruby and saved a file "calc.rb" per this lesson, which
is saved to the same folder as Ruby.

However, when I finish entering the code, and press "F5" to run it, I
get a message saying that "The system cannot find the file specified."

What am I doing wrong?

Thanks for your help.