docs: fix $(PKG)_SITE_METHOD names (UPPER CASE => lower case)

Currently, the documentation says $(PKG)_SITE_METHOD should have upper
case names such as SVN or GIT, but the implementation only accepts lower
case svn or git. This fixes the documentation.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>