fix typo in flag description (#3520)

examples/route_guide/client/client.go

@@ -40,7 +40,7 @@ var (
 	tls                = flag.Bool("tls", false, "Connection uses TLS if true, else plain TCP")
 	caFile             = flag.String("ca_file", "", "The file containing the CA root cert file")
 	serverAddr         = flag.String("server_addr", "localhost:10000", "The server address in the format of host:port")
-	serverHostOverride = flag.String("server_host_override", "x.test.youtube.com", "The server name use to verify the hostname returned by TLS handshake")
+	serverHostOverride = flag.String("server_host_override", "x.test.youtube.com", "The server name used to verify the hostname returned by the TLS handshake")
 )
 
 // printFeature gets the feature for the given point.