Fix typo referencing '17', and provide an example randomizer

Change-Id: I037323c17efd736ff59591200bcffeebd5450cdf

nodepool/nodepool.yaml

@@ -8,7 +8,12 @@ cron:
 # TODO: Please choose a random hour for nodepool image updates.
 # This will help reduce the load on upstream git farms & mirros where all 3rd
 # party ci systems start building images at the same time.
-# Doing so is easy: update the first '17' below with a random number between 0 to 23
+# Doing so is easy, run the following command and replace image-update string
+# with the results:
+#   echo "0 $((RANDOM%23+1)) * * *"
+# Example:
+#   root@jenkins:~# echo "0 $((RANDOM%23+1)) * * *"
+#   0 16 * * *
 # This references the hour of the day when images will be built.
   image-update: '0 0 * * *'