Fix typos in eval_inst.h (#6775)

toolchain/check/eval_inst.h

@@ -124,7 +124,7 @@ constexpr auto ConstantKindHasEvalConstantInst(SemIR::InstConstantKind kind)
 template <typename InstT, bool HasFn, bool HasInstId>
 struct FunctionTypeForEvalConstantInstImpl {
   // By default, we want no `EvalConstantInst` function at all. But we can't
-  // express that, so use the type `auto () -> voic` as a placaeholder.
+  // express that, so use the type `auto () -> void` as a placeholder.
   using Type = auto() -> void;
 };
 template <typename InstT>