build-system/gnu: Fix default name for the "doc" output directory.

* guix/build/gnu-build-system.scm (configure)[package-name]: Drop the
  prefix corresponding to the hash part of OUT.

guix/build/gnu-build-system.scm

@@ -109,9 +109,10 @@ makefiles."
            (base (basename out))
            (dash (string-rindex base #\-)))
       ;; XXX: We'd rather use `package-name->name+version' or similar.
-      (if dash
+      (string-drop (if dash
                        (substring base 0 dash)
-          base)))
+                       base)
+                   (+ 1 (string-index base #\-)))))
 
   (let* ((prefix     (assoc-ref outputs "out"))
          (bindir     (assoc-ref outputs "bin"))