doc/ref: fix HTML anchor link in spec

Anchor links are case sensitive.

Change-Id: I582cda98c3a63bf405fe15495fcb75e3aeb80e06
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9041
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

doc/ref/spec.md

@@ -1354,7 +1354,7 @@ Referencing a definition will recursively [close](#ClosedStructs) it.
 That is, a referenced definition will not unify with a struct
 that would add a field anywhere within the definition that it does not
 already define or explicitly allow with a pattern constraint or `...`.
-[Embeddings](#Embedding) allow bypassing this check.
+[Embeddings](#embedding) allow bypassing this check.
 
 If referencing a definition would always result in an error, implementations
 may report this inconsistency at the point of its declaration.