Add the weak count to the RC diagram

[ColinPitrat]

This adds the weak count in the RC diagram, and rename count to strong making it more accurate.

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[djmitche]

I considered this when updating the diagram, but this, too, is actually incorrect -- the weak count is 2 in this case (if I remember correctly from that PR). This gets into more implementation detail than we really want to address in this slide, so I chose to approximate the truth and stick to the topic at hand.

The text above this diagram is incorrect, though, in that it mentions strong and weak pointers when of course they are counters.

Maybe the best approach is removing mention of weak from the slide itself (and replacing "pointers" with "counters"), and adding a speaker note indicating that the diagram isn't telling the full story, perhaps with a link to the Rc implementation.