Clarify dead-end wording in `pgr_extractVertices` workshop section

[akronim26]

Problem
In the workshop section 2.3.2, during the updation of the geom and osm_id column, the verification step says:

Not expecting to be done due to the fact that some vertices are dead ends.

This wording is confusing. In this context the issue is that some nodes are dead ends, so some vertices will only ever be a source or only ever be a target, and therefore the UPDATE statements cannot fill geom and osm_id for every row.

Improvement

We can clarify the sentence like:

Do not expect this count to be the same as the total number of rows in the vertices table, because some nodes are dead ends: some vertices will only be sources and others only targets, so not all rows can be updated.

Alternatives

[akronim26]

Working on thia!