doc/ref/spec.md: fix spec bug and broken link

Change-Id: Ibb578957275ea26c73688d00d291c61e391622d7
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7162
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

doc/ref/spec.md

@@ -1243,8 +1243,8 @@ declaration is defined for all fields.
 Recursively closing a struct is equivalent to adding `..._|_` to its its root
 and any of its substructures that are not defined for all fields.
 
-Syntactically, structs are recursively closed explicitly with
-the `close` builtin or implicitly by [definitions](#Definitions).
+Syntactically, structs are closed explicitly with the `close` builtin or
+implicitly and recursively by [definitions](#definitions-and-hidden-fields).
 
 
 ```