config-linux.md: clearly require absolute path for namespace

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

config-linux.md

@@ -35,7 +35,7 @@ The following parameters can be specified to setup namespaces:
     * **`user`** the container will be able to remap user and group IDs from the host to local users and groups within the container.
     * **`cgroup`** the container will have an isolated view of the cgroup hierarchy.
 
-* **`path`** *(string, OPTIONAL)* - path to namespace file in the [runtime mount namespace](glossary.md#runtime-namespace)
+* **`path`** *(string, OPTIONAL)* - an absolute path to namespace file in the [runtime mount namespace](glossary.md#runtime-namespace)
 
 If a path is specified, that particular file is used to join that type of namespace.
 If a namespace type is not specified in the `namespaces` array, the container MUST inherit the [runtime namespace](glossary.md#runtime-namespace) of that type.