[SPARK-12211][DOC][GRAPHX] Fix version number in graphx doc for migration from 1.1

Migration from 1.1 section added to the GraphX doc in 1.2.0 (see https://spark.apache.org/docs/1.2.0/graphx-programming-guide.html#migrating-from-spark-11) uses \{{site.SPARK_VERSION}} as the version where changes were introduced, it should be just 1.2.

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #10206 from aray/graphx-doc-1.1-migration.

docs/graphx-programming-guide.md

@@ -70,7 +70,7 @@ operators (e.g., [subgraph](#structural_operators), [joinVertices](#join_operato
 
 ## Migrating from Spark 1.1
 
-GraphX in Spark {{site.SPARK_VERSION}} contains a few user facing API changes:
+GraphX in Spark 1.2 contains a few user facing API changes:
 
 1. To improve performance we have introduced a new version of
 [`mapReduceTriplets`][Graph.mapReduceTriplets] called