public: typo: use ' as apostrophe in grant_table.h

If grep 2.23 is installed, build fails like this:
...
mkdir -p compat
grep -v 'DEFINE_XEN_GUEST_HANDLE(long)' public/grant_table.h | \
python /home/SOURCES/xen/xen/xen.git/xen/tools/compat-build-source.py >compat/grant_table.c.new
mv -f compat/grant_table.c.new compat/grant_table.c
gcc  ... -o compat/grant_table.i compat/grant_table.c
compat/grant_table.c:33:1: error: unterminated comment
 /*
 ^
compat/grant_table.c:28:0: error: unterminated #ifndef
 #ifndef __XEN_PUBLIC_GRANT_TABLE_H__
 ^
Makefile:62: recipe for target 'compat/grant_table.i' failed
make[3]: *** [compat/grant_table.i] Error 1
rm compat/grant_table.c
make[3]: Leaving directory '/home/SOURCES/xen/xen/xen.git/xen/include'
...

This is because grant_table.h contains this (note the
apostrophe): "granter’s memory", and `grep -v', in version
2.23, stops processing the file (while, for instance,
until 2.22, this was not happening).

Although the above behavior is likely an issue in grep,
(https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22461)
I think we better switch to using " ' " in that line
anyway, as we do basically everywhere else (even in
the same file).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

xen/include/public/grant_table.h

@@ -43,7 +43,7 @@
  * table are identified by grant references. A grant reference is an
  * integer, which indexes into the grant table. It acts as a
  * capability which the grantee can use to perform operations on the
- * granter’s memory.
+ * granter's memory.
  *
  * This capability-based system allows shared-memory communications
  * between unprivileged domains. A grant reference also encapsulates