x86/speculation/mds: Fix documentation typo

Fix a minor typo in the MDS documentation: "eanbled" -> "enabled".

Reported-by: Jeff Bastian <jbastian@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>