Fix bug in VectorClock example

[stuartsan]

Great book, thanks for writing it!

I think there's a bug in the merge method in the VectorClock example; as-is the result of the concatenation will be something like:

['foo', 'bar', { 'foo': 1, 'bar': 2 }]

And I believe it should be more like:

['foo', 'bar', 'foo', 'bar']