Small formatting fix (#1866)

This is a typo, that actually @mgeisler stopped in my previous PR, I was
just not around to fix it fast enough to make it to the previous merge.
Thanks for the catch!

src/smart-pointers/box.md

@@ -82,7 +82,7 @@ fn main() {
 
 - 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
+  have to use indirection, a `Box` or reference of some kind, instead of storing
   the value directly.
 
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