fix typo [ci skip]

This error does not happen right now so this typo is
not serious, unless you locally edit insns.def.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64791 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/ruby_vm/helpers/scanner.rb

@@ -43,7 +43,7 @@ class RubyVM::Scanner
 
   def scan! re
     scan re or raise sprintf "parse error at %s:%d near:\n %s...", \
-        @__FILE__, @__LINE__, @str[pos, 32]
+        @__FILE__, @__LINE__, @str[@pos, 32]
   end
 
   def [] key