Correct example yaml

Without the space I get:

    FATA[0000] Invalid config: yaml: line 66: could not find expected ':'

Signed-off-by: Ian Campbell <ijc@docker.com>

docs/yaml.md

@@ -110,7 +110,7 @@ supported here is `"yaml"` which will output the yaml used to generate the image
 file:
 ```
   - path: etc/linuxkit.yml
-    metadata:yaml
+    metadata: yaml
 ```
 
 ## `trust`