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 <yangx.jy@fujitsu.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220513101637.216487-1-yangx.jy@fujitsu.com

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]