Documentation: refer to config RANDOMIZE_BASE for kernel address-space randomization

The config RANDOMIZE_SLAB does not exist, the authors probably intended to
refer to the config RANDOMIZE_BASE, which provides kernel address-space
randomization. They probably just confused SLAB with BASE (these two
four-letter words coincidentally share three common letters), as they also
point out the config SLAB_FREELIST_RANDOM as further randomization within
the same sentence.

Fix the reference of the config for kernel address-space randomization to
the config that provides that.

Fixes: 6e885594 ("Documentation: Add section about CPU vulnerabilities for Spectre")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20211230171940.27558-1-lukas.bulwahn@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/hw-vuln/spectre.rst

@@ -468,7 +468,7 @@ Spectre variant 2
    before invoking any firmware code to prevent Spectre variant 2 exploits
    using the firmware.
 
-   Using kernel address space randomization (CONFIG_RANDOMIZE_SLAB=y
+   Using kernel address space randomization (CONFIG_RANDOMIZE_BASE=y
    and CONFIG_SLAB_FREELIST_RANDOM=y in the kernel configuration) makes
    attacks on the kernel generally more difficult.