[SPARK-19585][DOC][SQL] Fix the cacheTable and uncacheTable api call in the doc

## What changes were proposed in this pull request?

https://spark.apache.org/docs/latest/sql-programming-guide.html#caching-data-in-memory
In the doc, the call spark.cacheTable(“tableName”) and spark.uncacheTable(“tableName”) actually needs to be spark.catalog.cacheTable and spark.catalog.uncacheTable

## How was this patch tested?
Built the docs and verified the change shows up fine.

Author: Sunitha Kambhampati <skambha@us.ibm.com>

Closes #16919 from skambha/docChange.