Fix layer 7 load balancers typo (donnemartin#317)

Author: shiyujiucsb

README.md

@@ -692,7 +692,7 @@ Layer 4 load balancers look at info at the [transport layer](#communication) to
 
 ### Layer 7 load balancing
 
-Layer 7 load balancers look at the [application layer](#communication) to decide how to distribute requests.  This can involve contents of the header, message, and cookies.  Layer 7 load balancers terminates network traffic, reads the message, makes a load-balancing decision, then opens a connection to the selected server.  For example, a layer 7 load balancer can direct video traffic to servers that host videos while directing more sensitive user billing traffic to security-hardened servers.
+Layer 7 load balancers look at the [application layer](#communication) to decide how to distribute requests.  This can involve contents of the header, message, and cookies.  Layer 7 load balancers terminate network traffic, reads the message, makes a load-balancing decision, then opens a connection to the selected server.  For example, a layer 7 load balancer can direct video traffic to servers that host videos while directing more sensitive user billing traffic to security-hardened servers.
 
 At the cost of flexibility, layer 4 load balancing requires less time and computing resources than Layer 7, although the performance impact can be minimal on modern commodity hardware.