docs: market bid example error

[ktehranchi]

I noticed an error with the example on adding a market bid- the example defines a variable cost without an explicit CostCurve structure (PieceWiseStepData).

The docs for set_variable_cost! specify that data::Union{Nothing, TimeSeriesData, CostCurve{PiecewiseIncrementalCurve}} . The example in the docs does not use a CostCurve and throws an error.

However when I was testing different CostCurves, I found only PiecewiseStepData would function properly. I updated the docs with what worked for me.

These are the three CostCurves I tested:

IncrementalCurve(PiecewiseStepData([1.0, 2.0, 3.0], [4.0, 6.0]), 20.0)
PiecewiseIncrementalCurve(20.0, [1.0, 2.0, 3.0], [4.0, 6.0])
PiecewiseStepData([1.0, 2.0, 3.0], [24.0, 26.0])

Error for PiecewiseIncrementalCurve case:

julia> set_variable_cost!(sys, generator, time_series_data)
ERROR: TypeError: in _process_set_cost, expected PiecewiseStepData, got Type{PiecewiseIncrementalCurve}
Stacktrace:
 [1] _process_set_cost(::Type{CostCurve{PiecewiseIncrementalCurve}}, ::Type{PiecewiseStepData}, sys::System, component::ThermalStandard, cost::Deterministic)
   @ PowerSystems ~/.julia/packages/PowerSystems/F94iA/src/models/cost_function_timeseries.jl:242
 [2] set_variable_cost!(sys::System, component::ThermalStandard, data::Deterministic)
   @ PowerSystems ~/.julia/packages/PowerSystems/F94iA/src/models/cost_function_timeseries.jl:265
 [3] top-level scope
   @ REPL[64]:1