[docs] [SPARK-6306] Readme points to dead link

The link to "Specifying the Hadoop Version" currently points to http://spark.apache.org/docs/latest/building-with-maven.html#specifying-the-hadoop-version.

The correct link is: http://spark.apache.org/docs/latest/building-spark.html#specifying-the-hadoop-version

Author: Theodore Vasiloudis <tvas@sics.se>

Closes #4999 from thvasilo/patch-1 and squashes the following commits:

c34aea8 [Theodore Vasiloudis] Fix dead link in Readme

README.md

@@ -85,7 +85,7 @@ storage systems. Because the protocols have changed in different versions of
 Hadoop, you must build Spark against the same version that your cluster runs.
 
 Please refer to the build documentation at
-["Specifying the Hadoop Version"](http://spark.apache.org/docs/latest/building-with-maven.html#specifying-the-hadoop-version)
+["Specifying the Hadoop Version"](http://spark.apache.org/docs/latest/building-spark.html#specifying-the-hadoop-version)
 for detailed guidance on building for a particular distribution of Hadoop, including
 building for particular Hive and Hive Thriftserver distributions. See also
 ["Third Party Hadoop Distributions"](http://spark.apache.org/docs/latest/hadoop-third-party-distributions.html)