support src in --secret

Signed-off-by: Victor Vieux <victorvieux@gmail.com>

cli/command/service/opts.go

@@ -182,7 +182,7 @@ func (o *SecretOpt) Set(value string) error {
 
 		value := parts[1]
 		switch key {
-		case "source":
+		case "source", "src":
 			spec.source = value
 		case "target":
 			tDir, _ := filepath.Split(value)

docs/reference/commandline/service_create.md

@@ -137,7 +137,7 @@ Create a service specifying the secret, target, user/group ID and mode:
 ```bash
 $ docker service create --name redis \
     --secret source=ssh-key,target=ssh \
-    --secret source=app-key,target=app,uid=1000,gid=1001,mode=0400 \
+    --secret src=app-key,target=app,uid=1000,gid=1001,mode=0400 \
     redis:3.0.6
 4cdgfyky7ozwh3htjfw0d12qv
 ```