Revise datasources.update in api-ref.md (#842)

* Update api-ref.md

Future release of Tableau Server will allow rename of datasource.  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.

* Update "Datasource" to "Data Source"

In response to dzucker feedback

Author: annematronic

docs/api-ref.md

@@ -674,9 +674,9 @@ A refreshed `DatasourceItem`.
 datasource.update(datasource_item)
 ```
 
-Updates the owner or project of the specified data source. Data source name cannot be updated.
+Updates the specified data source. 
 
-REST API: [Update Datasource](https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref.htm#update_data_source)
+REST API: [Update Data Source](https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref.htm#update_data_source)
 
 **Parameters**