Fix raise ArgumentError missing comma in Ci::JobsFinder

What does this MR do and why

The type guard in the constructor is missing a comma:

raise ArgumentError 'type must be a subclass of Ci::Processable' unless type < ::Ci::Processable

Without it, Ruby parses ArgumentError 'string' as a method call named ArgumentError, which does not exist, so the guard raises NoMethodError with a message about an undefined method rather than the ArgumentError it intends. Anyone passing a wrong type gets an error that points at the wrong problem.

How to verify

raise ArgumentError 'x'
# => NoMethodError: undefined method `ArgumentError'
raise ArgumentError, 'x'
# => ArgumentError: x

Spotted by itaruby(to be released), a Ruby type checker I'm working on.