Merge pull request #25569 from friism/fix-typo-in-security-doc

update intro to say there are four things to consider

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;