Change security externalIdentifierType in example (#716)

Update the Vulnerability class syntax example to change the
externalIdentifierType from securityOther to securityAdvisory for a
GitHub advisory because it references a well known advisory source.

Thanks @puerco for catching this.

Signed-off-by: Rose Judge <rose.judge@broadcom.com>

model/Security/Classes/Vulnerability.md

@@ -33,7 +33,7 @@ Specifies a vulnerability and its associated information.
     },
     {
       "type": "ExternalIdentifier",
-      "externalIdentifierType": "securityOther",
+      "externalIdentifierType": "securityAdvisory",
       "identifier": "GHSA-r9p9-mrjm-926w",
       "identifierLocator": "https://github.com/advisories/GHSA-r9p9-mrjm-926w"
     },