update intro to say there are four things to consider

Signed-off-by: Michael Friis <friism@gmail.com>

docs/security/security.md

@@ -12,7 +12,7 @@ weight =-99
 
 # Docker security
 
-There are three major areas to consider when reviewing Docker security:
+There are four major areas to consider when reviewing Docker security:
 
  - the intrinsic security of the kernel and its support for
    namespaces and cgroups;