Fix struct Person definition in json_schema.md (#671)

Author: evanwporter

docs/json_schema.md

@@ -202,7 +202,7 @@ You can mark fields as deprecated in the JSON schema using `rfl::Deprecated`. Th
 ```cpp
 struct Person {
   rfl::Deprecated<"Use 'full_name' instead.", "The person's first name", std::optional<std::string>>
-      std::optional<first_name>;
+      first_name;
   rfl::Description<"The person's full name", std::string> full_name;
   float salary;
 };