guix-build: Fix typo.

* guix-build.in (guix-build)[find-package]: Use `x', not `_', to avoid
  confusion with `gettext'.

guix-build.in

@@ -205,7 +205,7 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
       (match (find-packages-by-name name version)
         ((p)                                      ; one match
          p)
-        ((p _ ...)                                ; several matches
+        ((p x ...)                                ; several matches
          (format (current-error-port)
                  (_ "warning: ambiguous package specification `~a'~%")
                  request)