Update hadoop doc link to current (#244)

* Update hadoop doc link to current

The linked r2.7.2 section on passwordless ssh has outdated instructions using dsa.
Current docs show using rsa.

* address review comment - use specific version (r3.2.1)

Co-authored-by: Ed Coleman etcoleman <edcoleman@apache.org>

README.md

@@ -200,5 +200,5 @@ configuring, initializing, and starting your application.
 [ll]: https://github.com/apache/fluo-uno/blob/master/LICENSE
 [logo]: contrib/uno-logo.png
 [Muchos]: https://github.com/apache/fluo-muchos
-[ssh-docs]: https://hadoop.apache.org/docs/r2.7.2/hadoop-project-dist/hadoop-common/SingleCluster.html#Setup_passphraseless_ssh
+[ssh-docs]: https://hadoop.apache.org/docs/r3.2.1/hadoop-project-dist/hadoop-common/SingleCluster.html#Setup_passphraseless_ssh
 [Accumulo Proxy]: https://github.com/apache/accumulo-proxy