Pregel example to express single source shortest path from https://spark.apache.org/docs/latest/graphx-programming-guide.html#pregel-api does not work due to incorrect type. The reason is that `GraphGenerators.logNormalGraph` returns the graph with `Long` vertices. Fixing `val graph: Graph[Int, Double]` to `val graph: Graph[Long, Double]`.

Author: Alexander Ulanov <nashb@yandex.ru>

Closes #7695 from avulanov/SPARK-9380-pregel-doc and squashes the following commits:

c269429 [Alexander Ulanov] Pregel example type fix