doc/rcu: fix g_free_rcu() usage example

The first argument of g_free_rcu() is a pointer to a structure.  But
foo_reclaim is used as a function name in the previous example along
with &foo as a pointer to the structure being reclaimed.  Make the
example consistent with the previous one.
Signed-off-by: NSergey Fedorov <serge.fdrv@gmail.com>
Message-Id: <1444837604-13712-1-git-send-email-serge.fdrv@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/rcu.txt

@@ -128,7 +128,7 @@ The core RCU API is small:
         the callback function is g_free, in particular, g_free_rcu can be
         used.  In the above case, one could have written simply:
 
-            g_free_rcu(foo_reclaim, rcu);
+            g_free_rcu(&foo, rcu);
 
      typeof(*p) atomic_rcu_read(p);