Added missing Discriminator Object link to Table of Contents (3.0.md)

Author: prsn-uk

versions/3.0.md

@@ -52,6 +52,7 @@ An OpenAPI definition can then be used by documentation generation tools to disp
 		- [Tag Object](#tagObject)
 		- [Reference Object](#referenceObject)
 		- [Schema Object](#schemaObject)
+		- [Discriminator Object](#discriminatorObject)
 		- [XML Object](#xmlObject)
 		- [Security Scheme Object](#securitySchemeObject)
 		- [OAuth Flows Object](#oauthFlowsObject)