Fix an error in the unit-like pattern match example

Single-segment identifiers are bindings and don't work to match
against a Unit struct.

Author: ehuss

src/attributes/type_system.md

@@ -156,7 +156,7 @@ Because a tuple variant's constructor's [visibility] is reduced to be no greater
 > // Cannot match a non-exhaustive unit-like or tuple struct except by using
 > // braced struct syntax with a wildcard.
 > // This would compile as `let Token { .. } = token;`
-> let Token = token;
+> let upstream::Token = token;
 > // This would compile as `let Id { 0: id_number, .. } = id;`
 > let Id(id_number) = id;
 >