Fixes typo- space missing (#77)

Author: sarahsimpers

source/network-security.txt

@@ -124,7 +124,7 @@ before you deploy your first {+cluster+}. The {+service+} |vpc| |cidr|
 block must not overlap with the |cidr| block of any |vpc| you intend to
 peer to. {+service+} limits the number of MongoDB instances per |vpc|
 based on the |cidr| block. For example, a project with a |cidr| block of
-``/24`` is limited to the equivalent of 273-node replica sets.
+``/24`` is limited to the equivalent of 27 3-node replica sets.
 
 .. image:: /includes/images/vpc-vnet-peering.svg
    :alt: "An image representing how MongoDB Atlas VPC/VNet peering works."