Complete the options for 'docker plugin install'

Signed-off-by: Yanqiang Miao <miao.yanqiang@zte.com.cn>

docs/reference/commandline/plugin_install.md

@@ -23,6 +23,7 @@ Install a plugin
 Options:
       --alias string            Local name for plugin
       --disable                 Do not enable the plugin on install
+      --disable-content-trust   Skip image verification (default true)
       --grant-all-permissions   Grant all permissions necessary to run the plugin
       --help                    Print usage
 ```