docs: align README id index key encoding

[immanuwell]

In the first README example ID is uint32, but FromObject encodes with index.Uint64 while FromKey uses index.Uint32.
Stored key is 8 bytes, query key 4 bytes, they never match, so Get always misses and the Modify/Delete steps below it become no-ops.
index.Uint64(obj.ID) doesnt compile either

Fix: use index.Uint32 in FromObject so it matches FromKey and the field type

Repro:
insert &MyObject{ID: 1}, then tbl.Get(txn, IDIndex.Query(1)) returns found=false with index.Uint64, found=true with index.Uint32:

type MyObject struct {
	ID  uint32
	Foo string
}

var IDIndex = statedb.Index[*MyObject, uint32]{
	Name:       "id",
	FromObject: func(o *MyObject) index.KeySet { return index.NewKeySet(index.Uint64(uint64(o.ID))) },
	FromKey:    func(id uint32) index.Key { return index.Uint32(id) },
	Unique:     true,
}

// insert &MyObject{ID: 1}, commit, then:
_, _, found := tbl.Get(db.ReadTxn(), IDIndex.Query(1))
// found == false
// swap FromObject to index.Uint32(o.ID) and found == true

[pippolo84]

Thanks!