Update custom-prompting.mdx (#892)

Author: davidkonigsberg

fern/products/ask-fern/pages/configuration/custom-prompting.mdx

@@ -25,7 +25,7 @@ Customizing the system prompt gives you the ability to tailor the AI search resp
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   </Step>
   <Step title="Add in your custom prompts">
-    Insert your custom prompts in the `system-prompt` field. Anthropic has a [great guide](https://docs.anthropic.com/en/docs/prompt-engineering/system-prompts) that can help provide ideas and examples for your custom prompts. 
+    Insert your custom prompts in the `system-prompt` field. Anthropic has a [great guide](https://docs.claude.com/en/docs/claude-code/sdk/modifying-system-prompts) that can help provide ideas and examples for your custom prompts. 
     You can also leverage Fern's [system prompts](https://github.com/fern-api/fern-platform/blob/4e823037606106a293f6abff24a9606cdcfe31fb/packages/fern-docs/search-server/src/utils/system-prompt.ts#L22-L31) as a starting point. 
 
   </Step>