docs: reflect API change — regions now map to IPv6 address arrays (#1305)

The allow-listing guide previously stated that each region key in the JSON
object held a single IPv6 address.
Because the API now returns multiple addresses per region, step 5 has been
updated to specify that each region key maps to an array of all IPv6s for
that region.

Ref: checkly/checkly-backend#6587

Author: roa

site/content/docs/monitoring/allowlisting.md

@@ -42,7 +42,7 @@ curl https://api.checklyhq.com/v1/static-ips.txt
 curl https://api.checklyhq.com/v1/static-ipv6s
 ```
 
-5. A JSON object with regions as keys and an IPv6 as value.
+5. A JSON object with regions as keys and arrays of all IPv6s of that region as the value
 
 ```bash
 curl https://api.checklyhq.com/v1/static-ipv6s-by-region