Documentation/hw-vuln: Fix rST warning

The following warning:

   Documentation/admin-guide/hw-vuln/cross-thread-rsb.rst:92: ERROR: Unexpected indentation.

was introduced by commit 493a2c2d.  Fix it by placing everything in
the same paragraph and also use a monospace font.

Fixes: 493a2c2d

 ("Documentation/hw-vuln: Add documentation for Cross-Thread Return Predictions")
Reported-by: Stephen Rothwell <sfr@canb@auug.org.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>