Include `runBlocking()` in context and dispatchers tutorial code

[toboche]

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The texts in the "Dispatchers and threads" and "Unconfined vs confined dispatcher" sections explicitly mention the example code having runBlocking call somewhere but the sample code does not show the call. Because of that the reader hass to assume (or check the source code) where the runBlocking call happens.

I'd propose to include the call to runBlocking in the code samples, so that the readers don't have to guess where the actual runBlocking call is done.