reference: fix broken alias for DomainRegexp

An incorrect alias snuck into 152af63,
and DomainRegexp was aliased to the regex for digests (DigestRegexp).

This didn't affect this repository, as it didn't use the aliases and migrated
to the new module, but does affect user of the old module that depend on the
aliases.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c8c2bc2)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

reference/regexp_deprecated.go

@@ -20,7 +20,7 @@ var DigestRegexp = reference.DigestRegexp
 // Deprecated: use [reference.DomainRegexp].
 //
 // [RFC 6874]: https://www.rfc-editor.org/rfc/rfc6874.
-var DomainRegexp = reference.DigestRegexp
+var DomainRegexp = reference.DomainRegexp
 
 // IdentifierRegexp is the format for string identifier used as a
 // content addressable identifier using sha256. These identifiers