Update assistant.md

formatting

Author: jordanbean-msft

articles/ai-services/openai/how-to/assistant.md

@@ -67,7 +67,7 @@ An individual assistant can access up to 128 tools including [code interpreter](
 
 ### Files
 
-Files can be uploaded via Studio, or programmatically. The `file_ids` parameter is required to give tools like `code_interpreter` access to files. When using the File upload endpoint, you must have the `purpose` set to assistants to be used with the Assistants API.
+Files can be uploaded via Studio, or programmatically. The `file_ids` parameter is required to give tools like `code_interpreter` access to files. When using the File upload endpoint, you must have the `purpose` set to `assistants` to be used with the Assistants API.
 
 ## Assistants playground