Reformulate speaker notes regarding Box (#1819)

The first change is to reformulate the English in a way, that
emphasizes, that this is not a decision of the compiler, but the
impossibility of computing an infinite value (e.g. changed the language
from "not compute" to "would not be able to compute").

The second change is to fix the error message, of course the error
message from the compiler is "recursive withOUT indirection", as
"recursive with indirection" is actually what we want.

src/smart-pointers/box.md

@@ -74,15 +74,16 @@ fn main() {
     so only the pointer is moved.
 
 - If `Box` was not used and we attempted to embed a `List` directly into the
-  `List`, the compiler would not compute a fixed size of the struct in memory
-  (`List` would be of infinite size).
+  `List`, the compiler would not be able to compute a fixed size for the struct
+  in memory (the `List` would be of infinite size).
 
 - `Box` solves this problem as it has the same size as a regular pointer and
   just points at the next element of the `List` in the heap.
 
-- Remove the `Box` in the List definition and show the compiler error.
-  "Recursive with indirection" is a hint you might want to use a Box or
-  reference of some kind, instead of storing a value directly.
+- Remove the `Box` in the List definition and show the compiler error. We get
+  the message "recursive without indirection", because for data recursion, we
+  have to use indirection, a Box or reference of some kind, instead of storing
+  the value directly.
 
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