stable_kernel_rules.txt: Remove extra space after Cc:

Fixes the checkpatch.pl warning:
"WARNING: Use a single space after Cc:"

Very lazy hackers just copy&paste the stable tag from this
document. As it contains an extra space, checkpatch.pl will
not like the resulting patch...
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>