SSZ spec: Merkleization - Typo in wording

[booleanfunction]

In simple-serialize.md, within the Merkelization section it says:

We now define Merkleization hash_tree_root(value) of an object value recursively:

merkleize(pack(value)) if value is a basic object or a vector of basic objects.

This part should exclude bitvectors as they are dealt with separately.

A suggested wording to correct this typo could be:

merkleize(pack(value)) if value is a basic object or a vector of basic objects, excluding bitvectors.

Bug report filed, issue #1892

[booleanfunction]

PR created and merged to add clarification of the SSZ Bitlist[N] and Bitvector[N] types

i.e. such that the above suggested changed wasn't applicable because List[boolean,N] and Vector[boolean, N] types are valid (and distinct from Bitlist[N] and Bitvector[N], respectively).