[SPARK-21267][DOCS][MINOR] Follow up to avoid referencing programming-guide redirector

## What changes were proposed in this pull request?

Update internal references from programming-guide to rdd-programming-guide

See https://github.com/apache/spark-website/commit/5ddf243fd84a0f0f98a5193a207737cea9cdc083 and https://github.com/apache/spark/pull/18485#issuecomment-314789751

Let's keep the redirector even if it's problematic to build, but not rely on it internally.

## How was this patch tested?

(Doc build)

Author: Sean Owen <sowen@cloudera.com>

Closes #18625 from srowen/SPARK-21267.2.