fixing broken links

[Graham Percival]

Hi Francisco,

Thanks for fixing the broken link, but this still looks suspect:

-----
 An introduction to Guile/Scheme as used in LilyPond can be found in the
-Learning Manual, see @rlearning{Scheme tutorial}.
+Learning Manual, see @rextend{Scheme tutorial}.
------

Since it's in @rextend, surely it's not in the Learning manual any more?


BTW, this is a great example to all doc-writers why you shouldn't
specify the manual before a @ref.  Just give the @ref and let texinfo
handle it.

Cheers,
- Graham