KB-22839 - added correct sample product import csv link (#1034)

<!-- Ticket number or summary of work -->
# [KB-22839]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* Updated the broken 404 link for sample product import csv link with
the correct one in the migration docs

## Release notes draft
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.
Fixed the broken product import template link in the migration docs

ping @bc-terra 


[KB-22839]:
https://bigcommercecloud.atlassian.net/browse/KB-22839?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: chris-nowicki

docs/start/migration/3_testing.md

@@ -144,5 +144,5 @@ Track IDs of all created test resources during migration, and use API DELETE cal
 - [Using Postman with BigCommerce APIs](https://developer.bigcommerce.com/docs/start/api-clients/postman)  
 - [Community Thread: Data Validation Scripts & Tips](https://community.bigcommerce.com/t5/Developers/Best-ways-to-validate-product-data-before-import/td-p/146590)  
 - [WebDAV Overview and Usage](https://support.bigcommerce.com/s/article/File-Access-WebDAV)  
-- [Sample Product Import CSV](https://support.bigcommerce.com/s/article/Sample-Product-CSV-Import-File)  
+- [Sample Product Import CSV](https://storage.googleapis.com/bigcommerce-production-dev-center/Knowledge%20Base/Products/Import%20Export%20v3%20Guide/v3-product-import-template.csv)  
 - [API Accounts and OAuth Scopes](https://developer.bigcommerce.com/docs/start/authentication/api-accounts)