docs: clarify Custom Context Condensing Prompt is in pulldown menu

As suggested by @hannesrudolph, updated the text to specify that the Custom Context Condensing Prompt is found in a pulldown menu rather than just a field.

Author: roomote

docs/features/intelligent-context-condensing.mdx

@@ -107,7 +107,7 @@ You can customize the context reduction prompt to better suit your specific doma
 To customize the prompt:
 1. Go to Roo Code settings (<Codicon name="gear" /> icon)
 2. Navigate to the "Prompt" settings section
-3. Find the "Custom Context Condensing Prompt" field
+3. Find the "Custom Context Condensing Prompt" in the pulldown
 4. Enter your custom prompt that instructs the AI on how to preserve specific types of information
 
 For example, if you're working on a complex debugging session, you might add instructions like: