docs/manual: update documentation about applying patches

The Buildroot manual is currently wrong when describing how patches
are applied: it says that <packagename>-*.patch is applied, and that
<packagename>-<seq>-*.patch is the recommended name for patches. This
is incorrect, since we're now using just <seq>-*.patch. This commit
adjusts the documentation accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>