Documentation: filesystem: fix "Removed Sysctls" table

the "Removed Sysctls" section is a table - bring it alive with ReST.
Signed-off-by: Sheriff Esseson <sheriffesseson@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/xfs.rst

@@ -337,11 +337,12 @@ None at present.
 Removed Sysctls
 ===============
 
+=============================	=======
   Name				Removed
-  ----				-------
+=============================	=======
   fs.xfs.xfsbufd_centisec	v4.0
   fs.xfs.age_buffer_centisecs	v4.0
-
+=============================	=======
 
 Error handling
 ==============