init/Kconfig: Fix module signing document location

This was moved in commit 94e980cc ("Documentation/module-signing.txt:
convert to ReST markup") and was missed by commit 8c27ceff ("docs:
fix locations of several documents that got moved").

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>