Update docs/migrating_to_8.md

Co-authored-by: hasezoey <hasezoey@gmail.com>

Author: vkarpov15

docs/migrating_to_8.md

@@ -76,4 +76,4 @@ const doc = new TestModel();
 // In Mongoose 8, this type is `string | null | undefined`.
 // In Mongoose 7, this type is `string | undefined`
 doc.name;
-```
+```