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