docs: update Grok LLM default model to grok-3

Updates documentation to reflect the change from grok-3-beta to grok-3
as the default model for GrokLLMService, following the general availability
release of the grok-3 model.

Related: pipecat-ai/pipecat#4209

Author: github-actions[bot]

api-reference/server/services/llm/grok.mdx

@@ -93,7 +93,7 @@ from pipecat.services.xai.llm import GrokLLMService
 
 llm = GrokLLMService(
     api_key=os.getenv("XAI_API_KEY"),
-    model="grok-3-beta",
+    model="grok-3",
 )
 ```
 
@@ -105,7 +105,7 @@ from pipecat.services.xai.llm import GrokLLMService
 llm = GrokLLMService(
     api_key=os.getenv("XAI_API_KEY"),
     settings=GrokLLMService.Settings(
-        model="grok-3-beta",
+        model="grok-3",
         temperature=0.7,
         top_p=0.9,
         max_completion_tokens=1024,