Fix error in polymorphism slide

VL2.tex

@@ -98,7 +98,7 @@ We can even have more generic stuff like:
 \begin{haskellcode}
 f :: a -> b
 \end{haskellcode}
-Whatever the function does... it has something of one type and returns something of another type (\emph{not} the same). That's all we know.
+Whatever the function does... it has something of one type and returns something of another type (it could be the same type, but doesn't need to). That's all we know.
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