gnu: JACK2: rename package to "jack2".

* gnu/packages/audio.scm (jack-2): Change package name to "jack2" to
  distinguish from JACK1.

gnu/packages/audio.scm

@@ -191,7 +191,7 @@ (define-public jack-1
 
 (define-public jack-2
   (package (inherit jack-1)
-    (name "jack")
+    (name "jack2")
     (version "1.9.10")
     (source (origin
              (method url-fetch)