gnu: dkimproxy: Correct licence.

* gnu/packages/mail.scm (dkimproxy)[license]: Change to gpl2+.

gnu/packages/mail.scm

@@ -2014,7 +2014,7 @@ (define-public dkimproxy
 proxy for signing outgoing email, and an inbound proxy for verifying signatures
 of incoming email.  With Postfix, the proxies can operate as either
 @code{Before-Queue} or @code{After-Queue} content filters.")
-    (license gpl3+)))
+    (license gpl2+)))
 
 (define-public mb2md
   (package