Fix Demo

The output is of "concat example" is :  

Next(🍎)
Next(🍐)
Next(🍊)
Next(🐱)
Next(🐭)

If people do not use 'concat()', the result is the same, the demo cannot show the effect of 'concat()', which is "waiting for each sequence to terminate successfully before emitting elements from the next sequence"

so I add one line code for better demonstration.

Rx.playground/Pages/Mathematical_and_Aggregate_Operators.xcplaygroundpage/Contents.swift

@@ -61,6 +61,7 @@ example("concat") {
     
     variable.value = subject2
     
+    subject2.onNext("I would be ignored")
     subject2.onNext("🐱")
     
     subject1.onCompleted()