index: fix howto page link

This does not point to the howto sub-page, but to an unknown anchor
instead. Fix the link using :doc:.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

index.rst

@@ -27,7 +27,7 @@ How to Contribute ?
    Development <http://lists.infradead.org/mailman/listinfo/linux-amlogic>`__
 -  `Mailing list for U-Boot
    Development <https://groups.io/g/u-boot-amlogic>`__
--  Build & Use Linux (& see `How-To <#howto>`__)
+-  Build & Use Linux (& see :doc:`How-To <howto>`)
 
 ::