[MINOR][DOCS] Fix few typos in structured streaming doc

## What changes were proposed in this pull request?

Minor typo in `even-time`, which is changed to `event-time` and a couple of grammatical errors fix.

## How was this patch tested?

N/A - since this is a doc fix. I did a jekyll build locally though.

Author: Ramkumar Venkataraman <rvenkataraman@paypal.com>

Closes #17037 from ramkumarvenkat/doc-fix.