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

init/Kconfig

@@ -1752,7 +1752,7 @@ config MODULE_SIG
 	help
 	  Check modules for valid signatures upon load: the signature
 	  is simply appended to the module. For more information see
-	  Documentation/module-signing.txt.
+	  <file:Documentation/admin-guide/module-signing.rst>.
 
 	  Note that this option adds the OpenSSL development packages as a
 	  kernel build dependency so that the signing tool can use its crypto