docs/manual: fix formatting for LIBFOO_SVN_EXTERNAL

Commit 7dd27cbe5b9e (support/download: add support to exclude svn
externals) introduced an improperly formatted list item. That was
carried over with bf2d7f8f5395 (package/pkg-generic: don't download svn
externals by default).

Fix that.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/adding-packages-generic.txt

@@ -347,7 +347,7 @@ not and can not work as people would expect it should:
   Git LFS to store large files out of band.  This is only available for
   packages downloaded with git (i.e. when +LIBFOO_SITE_METHOD=git+).
 
-+ +LIBFOO_SVN_EXTERNALS+ can be set to +YES+ to create an archive with
+* +LIBFOO_SVN_EXTERNALS+ can be set to +YES+ to create an archive with
   the svn external references. This is only available for packages
   downloaded with subversion.