Pregel example type fix

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

docs/graphx-programming-guide.md

@@ -800,7 +800,7 @@ import org.apache.spark.graphx._
 // Import random graph generation library
 import org.apache.spark.graphx.util.GraphGenerators
 // A graph with edge attributes containing distances
-val graph: Graph[Int, Double] =
+val graph: Graph[Long, Double] =
   GraphGenerators.logNormalGraph(sc, numVertices = 100).mapEdges(e => e.attr.toDouble)
 val sourceId: VertexId = 42 // The ultimate source
 // Initialize the graph such that all vertices except the root have distance infinity.