Improve ruby `desc` snippet

Instead of dummy text, first argument for `describe` becomes a class name derived from a spec file name. Example:
file: `some_class_spec.rb`
describe argument: `SomeClass`

This works pretty much the same as getting the class name for ruby `cla` snippets.
The only difference is that the idiomatic '_spec' is removed from spec file name, before converting to class name.

Let me know if more explanation or examples are needed!