docs: clarify externalId format

Author: iru

use-cases/manual-compliance.md

@@ -77,7 +77,7 @@ For later usage, fetch `SYSDIG_AWS_EXTERNAL_ID` from one of the previously regis
 $ curl -s "https://<SYSDIG_SECURE_ENDPOINT>/api/cloud/v2/accounts/<AWS_ACCOUNT_ID>?includeExternalId=true" \
 --header "Authorization: Bearer <SYSDIG_SECURE_API_TOKEN>"
 ```
-From the resulting payload get the `externalId` attribute value.
+From the resulting payload get the `externalId` attribute value, it should be a 32character string mixed with letters and numbers with no dashes, ex.:`0ab697b38dec8fb0932903jasfh38309`
 
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