Fix variable name in DifferentialProgramming.md

[Kshitij09]

In Podcast Player example of DifferentialProgramming.md, struct Observation has variable named podcastState, while it has been referenced as state in the meanError function.

Changed it to podcastState.

Relevant code snippet:

struct Observation {
    var podcastState: PodcastState
    var userSpeed: Float
}

func meanError(for model: PodcastSpeedModel, _ observations: [Observation]) -> Float {
    var error: Float = 0
    for observation in observations {
        error += abs(model.prediction(for: observation.state) - observation.userSpeed)
    }
    return error / Float(observations.count)
}

[dan-zheng]

Thank you!

[dan-zheng]

@swift-ci Please smoke test and merge

[dan-zheng]

@swift-ci Please smoke test Linux