Fix comments about System Functions

This commit fixes comments about "System Functions".
 * Add a missing comment about System Functions in the header
 * Fix singular to plural like others

Change-Id: I3feb94cd11a6683ca80093574d60fdf7420e3af2
diff --git a/functions-common b/functions-common
index d92e39c..eb9b4ac 100644
--- a/functions-common
+++ b/functions-common
@@ -15,6 +15,7 @@
 # - Process Functions
 # - Python Functions
 # - Service Functions
+# - System Functions
 #
 # The following variables are assumed to be defined by certain functions:
 #
@@ -1280,8 +1281,8 @@
 }
 
 
-# System Function
-# ===============
+# System Functions
+# ================
 
 # Only run the command if the target file (the last arg) is not on an
 # NFS filesystem.