vm_core.h: fix comment

* vm_core.h (rb_proc_t): fix comment.  safe_level is upto 3 now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46887 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -738,7 +738,7 @@ typedef struct {
 
     VALUE envval;		/* for GC mark */
     VALUE blockprocval;
-    int8_t safe_level;		/* 0..4 */
+    int8_t safe_level;		/* 0..3 */
     int8_t is_from_method;	/* bool */
     int8_t is_lambda;		/* bool */
 } rb_proc_t;