fix: wrong type of payload and headers in MessageExample

Rollback headers to Map[string, any]

Author: Pakisan

spec/asyncapi.md

@@ -1453,7 +1453,7 @@ Message Example Object represents an example of a [Message Object](#messageObjec
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="messageExampleObjectHeaders"></a>headers | `any` | The value of this field MUST validate against the [Message Object's headers](#messageObjectHeaders) field.
+<a name="messageExampleObjectHeaders"></a>headers | `Map[string, any]` | The value of this field MUST validate against the [Message Object's headers](#messageObjectHeaders) field.
 <a name="messageExampleObjectPayload"></a>payload | `any` | The value of this field MUST validate against the [Message Object's payload](#messageObjectPayload) field.
 <a name="messageExampleObjectName"></a>name | `string` | A machine-friendly name.
 <a name="messageExampleObjectSummary"></a>summary | `string` |  A short summary of what the example is about.