ssh/agent: fix example

Since there is no ClientAgent identifier, ExampleClientAgent wasn't
displayed as an example by godoc.

Change-Id: Ied8df7d8412c4aa483d42315986af49ba8fdbcef
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/208480
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

ssh/agent/example_test.go

@@ -13,20 +13,20 @@ import (
 	"golang.org/x/crypto/ssh/agent"
 )
 
-func ExampleClientAgent() {
-	// ssh-agent has a UNIX socket under $SSH_AUTH_SOCK
+func ExampleNewClient() {
+	// ssh-agent(1) provides a UNIX socket at $SSH_AUTH_SOCK.
 	socket := os.Getenv("SSH_AUTH_SOCK")
 	conn, err := net.Dial("unix", socket)
 	if err != nil {
-		log.Fatalf("net.Dial: %v", err)
+		log.Fatalf("Failed to open SSH_AUTH_SOCK: %v", err)
 	}
+
 	agentClient := agent.NewClient(conn)
 	config := &ssh.ClientConfig{
-		User: "username",
+		User: "gopher",
 		Auth: []ssh.AuthMethod{
-			// Use a callback rather than PublicKeys
-			// so we only consult the agent once the remote server
-			// wants it.
+			// Use a callback rather than PublicKeys so we only consult the
+			// agent once the remote server wants it.
 			ssh.PublicKeysCallback(agentClient.Signers),
 		},
 		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
@@ -34,8 +34,8 @@ func ExampleClientAgent() {
 
 	sshc, err := ssh.Dial("tcp", "localhost:22", config)
 	if err != nil {
-		log.Fatalf("Dial: %v", err)
+		log.Fatal(err)
 	}
-	// .. use sshc
+	// Use sshc...
 	sshc.Close()
 }