[SPARK-8639] [DOCS] Fixed Minor Typos in Documentation

Ticket: [SPARK-8639](https://issues.apache.org/jira/browse/SPARK-8639)

fixed minor typos in docs/README.md and docs/api.md

Author: Rosstin <asterazul@gmail.com>

Closes #7046 from Rosstin/SPARK-8639 and squashes the following commits:

6c18058 [Rosstin] fixed minor typos in docs/README.md and docs/api.md