Fix variable name in `DifferentialProgramming.md`
Fix variable name in `DifferentialProgramming.md`

In Podcast Player example of Intelligent Systems, struct `Observation` has var `podcastState`, while it has been referenced as `state` in the following cell. Changed it to `podcastState`.
nathawes/swift,nathawes/swift,roambotics/swift,hooman/swift,ahoppen/swift,rudkx/swift,JGiola/swift,xwu/swift,allevato/swift,hooman/swift,harlanhaskins/swift,glessard/swift,stephentyrone/swift,tkremenek/swift,atrick/swift,atrick/swift,nathawes/swift,glessard/swift,rudkx/swift,JGiola/swift,CodaFi/swift,CodaFi/swift,parkera/swift,allevato/swift,benlangmuir/swift,parkera/swift,nathawes/swift,stephentyrone/swift,stephentyrone/swift,jmgc/swift,harlanhaskins/swift,hooman/swift,hooman/swift,allevato/swift,stephentyrone/swift,atrick/swift,JGiola/swift,jckarter/swift,benlangmuir/swift,glessard/swift,ahoppen/swift,roambotics/swift,hooman/swift,aschwaighofer/swift,CodaFi/swift,roambotics/swift,stephentyrone/swift,glessard/swift,benlangmuir/swift,tkremenek/swift,aschwaighofer/swift,atrick/swift,jckarter/swift,nathawes/swift,benlangmuir/swift,CodaFi/swift,gregomni/swift,allevato/swift,hooman/swift,stephentyrone/swift,harlanhaskins/swift,apple/swift,airspeedswift/swift,harlanhaskins/swift,roambotics/swift,JGiola/swift,parkera/swift,jckarter/swift,xwu/swift,tkremenek/swift,roambotics/swift,benlangmuir/swift,parkera/swift,harlanhaskins/swift,xwu/swift,jmgc/swift,allevato/swift,ahoppen/swift,gregomni/swift,airspeedswift/swift,aschwaighofer/swift,airspeedswift/swift,jmgc/swift,xwu/swift,apple/swift,jmgc/swift,aschwaighofer/swift,nathawes/swift,CodaFi/swift,atrick/swift,aschwaighofer/swift,allevato/swift,rudkx/swift,rudkx/swift,airspeedswift/swift,xwu/swift,apple/swift,aschwaighofer/swift,rudkx/swift,parkera/swift,stephentyrone/swift,parkera/swift,parkera/swift,harlanhaskins/swift,jmgc/swift,ahoppen/swift,JGiola/swift,allevato/swift,JGiola/swift,glessard/swift,roambotics/swift,atrick/swift,benlangmuir/swift,gregomni/swift,tkremenek/swift,aschwaighofer/swift,tkremenek/swift,jckarter/swift,gregomni/swift,rudkx/swift,jckarter/swift,airspeedswift/swift,jckarter/swift,hooman/swift,airspeedswift/swift,xwu/swift,glessard/swift,jckarter/swift,harlanhaskins/swift,apple/swift,parkera/swift,xwu/swift,tkremenek/swift,gregomni/swift,jmgc/swift,nathawes/swift,CodaFi/swift,ahoppen/swift,ahoppen/swift,airspeedswift/swift,gregomni/swift,jmgc/swift,apple/swift,CodaFi/swift,tkremenek/swift,apple/swift