[SPARK-3378] [DOCS] Replace the word "SparkSQL" with right word "Spark SQL"

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2251 from sarutak/SPARK-3378 and squashes the following commits:

0bfe234 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-3378
bb5938f [Kousuke Saruta] Replaced rest of "SparkSQL" with "Spark SQL"
6df66de [Kousuke Saruta] Replaced "SparkSQL" with "Spark SQL"