add synGetTableColumns to Tables vignette

Description

https://r-docs.synapse.org/articles/tables.html

I suggest adding this little code chunk under - Tables to explain the synGetTableColumns function

  1.  

    1.  

      1. Changing Table Structure
        Get the existing Table schema as a list with synGetTableColumns().

```
existingSchema <- synGetTableColumns("synID")$asList()
```

Environment

None

Status

Assignee

Kimyen Truong

Reporter

Kelsey Montgomery

Labels

Validator

Kelsey Montgomery

Release Version History

None

Sprint

None

Fix versions

Priority

Major
Configure