coccinelle: misc: update uninitialized_var.cocci documentation

Remove the documentation link from the warning message because commit
3942ea7a ("deprecated.rst: Remove now removed uninitialized_var")
removed the section from documentation. Update the rule documentation
accordingly.

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>