DEVDOCS-6399 - Update authentication.yaml (#956)

Found error in api path. corrected. previously `/api/v3/io/` now
`/api/io/`

<!-- Ticket number or summary of work -->
# [DEVDOCS-6399]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* 

## Release notes draft
* Fixed server path for S2S Authentication in B2B Edition APIs

## Anything else?
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ping { @bc-Vince }


[DEVDOCS-6399]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6399?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-terra

docs/b2b-edition/specs/api-v3/authentication.yaml

@@ -19,7 +19,7 @@ info:
     email: [email protected]
   version: v3
 servers:
-  - url: "https://api-b2b.bigcommerce.com/api/v3/io"
+  - url: "https://api-b2b.bigcommerce.com/api/io"
 security:
   - authToken: []
 tags: