Update specs/simple-serialize.md

Co-Authored-By: dankrad <dankrad@ethereum.org>

specs/simple-serialize.md

@@ -45,6 +45,8 @@ This is a **work in progress** describing typing, serialization and Merkleizatio
 * **union**: union type containing one of the given subtypes
     * round bracket notation `(type1, type2, ...)`, e.g. `("uint64", "null")`
 
+#### Variable-size and fixed-size
+
 We recursively define "variable-size" types to be lists and unions and all types that contain a variable-size type. All other types are said to be "fixed-size".
 
 #### Illegal empty composites