Tweak some graph examples for clarity

[hjoliver]

We have (or soon will have, in new optional outputs docs etc.) a bunch of examples with triggers that spawn child tasks with the same name as the trigger:

foo:x => x

It's obvious what this means to the Cylc team, but it could be confusing to novice users - it kinda looks like x might be the same thing on both sides of the dependency.

To avoid anyone getting the wrong idea, change to (e.g.):

foo:x => bar
# or 
foo:x => bar_x