docs: Replace non-UTF-8 character in hypfs-paths.pandoc

docs: Replace non-UTF-8 character in hypfs-paths.pandoc

From the docs cronjob on xenbits:

  /usr/bin/pandoc --number-sections --toc --standalone misc/hypfs-paths.pandoc --output html/misc/hypfs-paths.html
  pandoc: Cannot decode byte '\x92': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream
  make: *** [Makefile:236: html/misc/hypfs-paths.html] Error 1

Fixes: 5a4a411bde4 ("docs: specify stability of hypfs path documentation")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>