doc-rst: fix inline emphasis in unshare.rst

The asterisk of the pointer is interpreted as a start tag for inline
emphasis. Asterisks which are not Sphinx markup need to be quoted in
rst-files. This fixes the Sphinx warning:

Documentation/userspace-api/unshare.rst:108: WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>