doc: zh_CN: index files in filesystems subdirectory

Add filesystems subdirectory into the table of Contents for zh_CN,
all translations residing on it would be indexed conveniently.

Reviewed-by: Alex Shi <[email protected]>
Reviewed-by: Mauro Carvalho Chehab <[email protected]>
Signed-off-by: Wang Wenhu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 53f46a8..e7b46da 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -1,3 +1,5 @@
+.. _filesystems_index:
+
 ===============================
 Filesystems in the Linux kernel
 ===============================