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>

Documentation/stable_kernel_rules.txt

@@ -93,7 +93,7 @@ format in the sign-off area:
 Also, some patches may have kernel version prerequisites.  This can be
 specified in the following format in the sign-off area:
 
-     Cc:  <stable@vger.kernel.org> # 3.3.x-
+     Cc: <stable@vger.kernel.org> # 3.3.x-
 
    The tag has the meaning of:
      git cherry-pick <this commit>