Update index.md (#4159)

docs/mkdocs/docs/api/basic_json/index.md

@@ -14,7 +14,7 @@ template<
     template<typename U> class AllocatorType = std::allocator,
     template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
     class BinaryType = std::vector<std::uint8_t>,
-    class CustomBaseClass = void>
+    class CustomBaseClass = void
 >
 class basic_json;
 ```