Revise datasources.update in api-ref.md

[annematronic]

Two Changes:

1. Remove note about rename of datasources not being possible, as a future release of Tableau Server will allow rename of datasource.
2. Remove details about what can be changed about a datasource, as it is out of date (more than just project and owner can be changed).

In general, I believe its best to let the official REST API documentation (https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref.htm) be the official source of what the REST API can and can't do (correct me if I'm wrong). The REST API docs will be updated prior to the server release.

[dzucker-tab]

The link to "REST API: [Update Datasource]" should be "REST API: [Update Data Source]"

[annematronic]

The link to "REST API: [Update Datasource]" should be "REST API: [Update Data Source]"

Fixed this

[dzucker-tab]

I approve Anne's change, but think I am not configured as an approver.