docs: update README docs reference to match current page structure

The writing-prompts page was removed in the docs reorg. Replace
"prompt writing guide" with "format spec" and "CLI reference" to
reflect the actual doc pages users will find.

Co-authored-by: Ralphify <noreply@ralphify.co>

Author: 2 people

README.md

@@ -180,7 +180,7 @@ Ralphs installed by agr go to `.agents/ralphs/` and are automatically discovered
 
 ## Documentation
 
-Full documentation at **[ralphify.co/docs](https://ralphify.co/docs/)** — getting started tutorial, prompt writing guide, cookbook, and troubleshooting.
+Full documentation at **[ralphify.co/docs](https://ralphify.co/docs/)** — getting started tutorial, format spec, cookbook, CLI reference, and troubleshooting.
 
 ## Requirements