Semantics fix

[jd-lara]

Step length? -> Interval

https://github.com/NREL-SIIP/Examples/blob/1387349bcb729d7fdedaf02309022cfeb1578ba9/script/PowerSimulations.jl%20Examples/sequential_simulations.jl#L40

[claytonpbarrows]

I like interval.

[jd-lara]

the comment says:

We can create a vector of initial times for a system by defining the step length and the horizon.

Then it should be

We can create a vector of initial times for a system by defining the interval and the horizon.