Fix a link to the spec, which has been moved to a new folder

docs/docs/dev/index.md

@@ -8,4 +8,4 @@ So you already read the [User Manual](../index.md) and still want to learn more
 
 The developer documentation can help you with that. It is a collection of materials expanding on _using_ into _understanding_ Hydra. It is aimed anyone wanting to get a deeper understanding of the Hydra protocol, protocol architects who want to build their own variants and of course developers who contribute to the [reference implementation](https://github.com/input-output-hk/hydra) of Hydra itself.
 
-Consequently, the following sections are more technical and maybe not as cohesive as the user manual, but the [architecture](./architecture) or the [specification](./specification) should be a good starting point. 
+Consequently, the following sections are more technical and maybe not as cohesive as the user manual, but the [architecture](./architecture) or the [specification](/docs/core-concepts/specification.md) should be a good starting point.