[SPARK-16911] Fix the links in the programming guide

## What changes were proposed in this pull request?

 Fix the broken links in the programming guide of the Graphx Migration and understanding closures

## How was this patch tested?

By running the test cases  and checking the links.

Author: Shivansh <shiv4nsh@gmail.com>

Closes #14503 from shiv4nsh/SPARK-16911.