fix: allow reusability of Server Variable Objects

Refs asyncapi#775

spec/asyncapi.md

@@ -359,7 +359,7 @@ Field Name | Type | Description
 <a name="serverObjectProtocol"></a>protocol | `string` | **REQUIRED**. The protocol this URL supports for connection. Supported protocol include, but are not limited to: `amqp`, `amqps`, `http`, `https`, `ibmmq`, `jms`, `kafka`, `kafka-secure`, `anypointmq`, `mqtt`, `secure-mqtt`, `solace`, `stomp`, `stomps`, `ws`, `wss`, `mercure`.
 <a name="serverObjectProtocolVersion"></a>protocolVersion | `string` | The version of the protocol used for connection. For instance: AMQP `0.9.1`, HTTP `2.0`, Kafka `1.0.0`, etc.
 <a name="serverObjectDescription"></a>description | `string` | An optional string describing the host designated by the URL. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
-<a name="serverObjectVariables"></a>variables | Map[`string`, [Server Variable Object](#serverVariableObject)] | A map between a variable name and its value.  The value is used for substitution in the server's URL template.
+<a name="serverObjectVariables"></a>variables | Map[`string`, [Server Variable Object](#serverVariableObject) \| [Reference Object](#referenceObject)]] | A map between a variable name and its value.  The value is used for substitution in the server's URL template.
 <a name="serverObjectSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used with this server. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a connection or operation.
 <a name="serverObjectBindings"></a>bindings | [Server Bindings Object](#serverBindingsObject) \| [Reference Object](#referenceObject) | A map where the keys describe the name of the protocol and the values describe protocol-specific definitions for the server.