docs/xsplice: Fix syntax when compiling to pdf with pandoc

docs/xsplice: Fix syntax when compiling to pdf with pandoc

Pandoc (version 1.12.4.2 from Debian Jessie) complains at the embedded \n in
the signature checking paragraph.

  /usr/bin/pandoc --number-sections --toc --standalone misc/xsplice.markdown
    --output pdf/misc/xsplice.pdf
  ! Undefined control sequence.
  l.1085 appended\textasciitilde{}\n

Surround the string in backticks to make it verbatim text.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>