docs: tweaks rate limit doc on model (envoyproxy#318)

**Commit Message**

The model name is extracted by AI Gateway filter, not the one explicitly
added by downstream clients.


**Related Issues/PRs (if applicable)**

Follow up on envoyproxy#260

---------

Signed-off-by: Takeshi Yoneda <t.y.mathetake@gmail.com>
Signed-off-by: Loong <long0dai@foxmail.com>

site/docs/capabilities/usage-based-ratelimiting.md

@@ -14,7 +14,7 @@ This guide focuses on AI Gateway's specific capabilities for token-based rate li
 AI Gateway leverages Envoy Gateway's Global Rate Limit API to provide token-based rate limiting for LLM requests. Key features include:
 - Token usage tracking based on model and user identifiers
 - Configuration for tracking input, output, and total token metadata from LLM responses
-- Model-specific rate limiting using AI Gateway headers (`x-ai-eg-model`)
+- Model-specific rate limiting using AI Gateway headers (`x-ai-eg-model`) which is inserted by the AI Gateway filter with the model name extracted from the request body.
 - Support for custom token cost calculations using CEL expressions
 
 ## Token Usage Behavior
@@ -151,18 +151,17 @@ When configuring rate limits:
 
 For proper cost control and rate limiting, requests must include:
 - `x-user-id`: Identifies the user making the request
-- `x-ai-eg-model`: Identifies the model being used
 
 Example request:
 ```shell
 curl --fail \
     -H "Content-Type: application/json" \
     -H "x-user-id: user123" \
-    -H "x-ai-eg-model: gpt-4" \    # Both user ID and model are required
     -d '{
         "messages": [
             {
                 "role": "user",
+                "model": "gpt-4",
                 "content": "Hello!"
             }
         ]