[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.