Fix "Building Spark With Maven" link in README.md

Corrected link to the Building Spark with Maven page from its original (http://spark.apache.org/docs/latest/building-with-maven.html) to the current page (http://spark.apache.org/docs/latest/building-spark.html)

Author: Denny Lee <denny.g.lee@gmail.com>

Closes #3802 from dennyglee/patch-1 and squashes the following commits:

15f601a [Denny Lee] Update README.md

(cherry picked from commit 08b18c7eb790c65670778eab8a6e32486c5f76e9)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>