x86/speculation: Add missing srbds=off to the mitigations= help text

The mitigations= cmdline option help text misses the srbds=off option.
Add it.

  [ bp: Add a commit message. ]

Signed-off-by: Xiao Yang <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: yangx-jy

Documentation/admin-guide/kernel-parameters.txt

@@ -3103,6 +3103,7 @@
 					       mds=off [X86]
 					       tsx_async_abort=off [X86]
 					       kvm.nx_huge_pages=off [X86]
+					       srbds=off [X86,INTEL]
 					       no_entry_flush [PPC]
 					       no_uaccess_flush [PPC]