sched_ext: Fix nonexistent field in sched-ext.rst example

The ops.exit() example in sched-ext.rst reads ei->type, but
struct scx_exit_info has never had a type field - the exit reason is
exposed as ei->kind since the struct was introduced. A scheduler
written following the example fails to compile with

    error: no member named 'type' in 'struct scx_exit_info'

Use ei->kind.

Fixes: fa48e8d2c7 ("sched_ext: Documentation: scheduler: Document extensible scheduler class")
Signed-off-by: Liang Luo <luoliang@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/scheduler/sched-ext.rst

@@ -230,7 +230,7 @@ optional. The following modified excerpt is from
 
     void BPF_STRUCT_OPS(simple_exit, struct scx_exit_info *ei)
     {
-            exit_type = ei->type;
+            exit_type = ei->kind;
     }
 
     SEC(".struct_ops")