afs: Documentation: correct reference to CONFIG_AFS_FS

Commit 0795e7c0 ("[AFS]: Update the AFS fs documentation.") adds a new
section listing the build configuration options that need to be enabled for
the AFS file system.

The documentation refers to CONFIG_AFS, but the option is called
CONFIG_AFS_FS, since the beginning of Linux's git history.

Refer to the config option with the correct name.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230720094301.9888-1-lukas.bulwahn@gmail.com

Documentation/filesystems/afs.rst

@@ -44,7 +44,7 @@ options::
 
 	CONFIG_AF_RXRPC		- The RxRPC protocol transport
 	CONFIG_RXKAD		- The RxRPC Kerberos security handler
-	CONFIG_AFS		- The AFS filesystem
+	CONFIG_AFS_FS		- The AFS filesystem
 
 Additionally, the following can be turned on to aid debugging::