Update Speaker Notes for compound-types.md (#207)

It could be a "gotcha" moment for readers when they associate `()` with `void` from, say, C++.

src/basic-syntax/compound-types.md

@@ -56,5 +56,7 @@ Tuples:
   the only valid value of that type - that is to say both the type and its value
   are expressed as `()`. It is used to indicate, for example, that a function or
   expression has no return value, as we'll see in a future slide. 
+    * You can think of it as `void` that can be familiar to you from other 
+      programming languages.
 
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