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>

Documentation/userspace-api/unshare.rst

@@ -107,7 +107,7 @@ the benefits of this new feature can exceed its cost.
 
 unshare() reverses sharing that was done using clone(2) system call,
 so unshare() should have a similar interface as clone(2). That is,
-since flags in clone(int flags, void *stack) specifies what should
+since flags in clone(int flags, void \*stack) specifies what should
 be shared, similar flags in unshare(int flags) should specify
 what should be unshared. Unfortunately, this may appear to invert
 the meaning of the flags from the way they are used in clone(2).