update global headers example code

Author: devalog

fern/products/sdks/guides/configure-global-headers.mdx

@@ -99,16 +99,19 @@ Alternatively, you can add headers to the `api` block in your `generators.yml` f
 api:
   - openapi: ./path/to/openapi
     headers: 
-      X-Version: 
-        name: version
-        type: literal<"1234">
+      custom_api_key:
+        name: api_key
+        type: string
+      userpool_id:
+        name: userpool_id
+        type: optional<string>
 ```
 
 For more information, see the [`generators.yml` reference documentation](/sdks/reference/generators-yml#apiheaders).
 
-### Client code
+#### Client con
 
-This configuration yields the following client:
+Both of the above configurations produces the same client code:
 
 ```python
 import os
@@ -117,5 +120,7 @@ class Client:
 
   def __init__(self, *, apiKey: str, userpoolId: typing.Optional[str])
 ```
+
+
 </Accordion>
 </AccordionGroup>