fix some warnings

src/json.hpp.re2c

@@ -372,7 +372,7 @@ contains a `mapped_type`, whereas `std::vector` fails the test.
     public:                                                                   \
         static constexpr bool value =                                         \
                 std::is_integral<decltype(detect(std::declval<T>()))>::value; \
-    };
+    }
 
 NLOHMANN_JSON_HAS_HELPER(mapped_type);
 NLOHMANN_JSON_HAS_HELPER(key_type);