doc: Use 'define-public' in example package recipe.

* doc/guix.texi (Defining Packages): Use 'define-public' instead of
  'define'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -1371,7 +1371,7 @@ package looks like this:
   #:use-module (guix build-system gnu)
   #:use-module (guix licenses))
 
-(define hello
+(define-public hello
   (package
     (name "hello")
     (version "2.8")