Fix typo in proposal rationale. (#6427)

proposals/p4864.md

@@ -99,7 +99,7 @@ clarification:
         different local interpretation.
 -   [Interoperability with and migration from existing C++ code](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code)
     -   All C++ identifiers are nameable from Carbon code without conflicts.
-        Virtual functions introduced in Carbon can be overridden in Carbon
+        Virtual functions introduced in C++ can be overridden in Carbon
         regardless of their name. C++ code can be migrated to Carbon even if its
         name in C++ has special meaning in Carbon.
 -   [Principle: Prefer providing only one way to do a given thing](/docs/project/principles/one_way.md)