Fix wrong description of multi-block tipsets

[jsoares]

The expected consensus protocol provides an expected number of blocks per tipset, not an upper bound. This is clear from the section on tipsets, or from looking at a chain explorer, but is misrepresented in this part.

The actual expected number (5) does not seem relevant in this context either, so I took it out rather than rephrasing.

@xuechao2 and @sa8 noticed the issue.