docker upgrade command fix (#1044)

`apt-get upgrade` doesn't take an argument. To upgrade docker specifically would require `apt-get install docker-engine`.
docker · Jan 6, 2017 · 7a5c3dd · 7a5c3dd
1 parent d49d1e3
commit 7a5c3dd
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/engine/installation/linux/ubuntulinux.md b/engine/installation/linux/ubuntulinux.md
@@ -596,7 +596,7 @@ updates Docker if a new version is available.
 
 ```bash
 $ sudo apt-get update
-$ sudo apt-get upgrade docker-engine
+$ sudo apt-get install docker-engine
 ```
 
 ## Uninstallation