Fix custom demos

[jneen]

In working on the new Rouge plugin example codebase, I discovered that demo handling has been broken for some time, on account of a missed ||=. This should work, but is broken on master:

class MyCoolLexer < Rouge::RegexLexer
  demo <<~MY_DEMO
    here is my demo text
  MY_DEMO
end

This will try to load a file from the Rouge install location itself, ignoring the directive in the class.

[tancnle]

Thanks, @jneen. LGTM 🚀