afs: Documentation: correct reference to CONFIG_AFS_FS

Commit 0795e7c031c4 ("[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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/filesystems/afs.rst

···
 
 	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::