core/br2-external: fix link to manual for converting old trees
To be noted: that link will only be valid once we have a released
manual. In the meantime, it's accessible on the nightly manual:
http://nightly.buildroot.org/#br2-external-converting
Reported-by: Benoît Allard <benoit.allard@greenbone.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Benoît Allard <benoit.allard@greenbone.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>