Documentation/hw-vuln: Update spectre doc

commit 7c693f54 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")

adds the "ibrs " option  in
Documentation/admin-guide/kernel-parameters.txt but omits it to
Documentation/admin-guide/hw-vuln/spectre.rst, add it.
Signed-off-by: Lin Yujun <linyujun809@huawei.com>
Link: https://lore.kernel.org/r/20220830123614.23007-1-linyujun809@huawei.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

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

@@ -613,6 +613,7 @@ kernel command line.
                 eibrs                   enhanced IBRS
                 eibrs,retpoline         enhanced IBRS + Retpolines
                 eibrs,lfence            enhanced IBRS + LFENCE
+                ibrs                    use IBRS to protect kernel
 
 		Not specifying this option is equivalent to
 		spectre_v2=auto.