string.c (rb_str_format_m): Fix the example code of the doc

Change `%08x` to `%016x` because of two reasons:

* `%016x` demonstrates that we can use two or more digits here.
* Currently, many people uses 64-bit environment.
  (I'm unsure if object_id is a good example here, though...)
I'm unsure if

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62091 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -1988,7 +1988,7 @@ rb_str_times(VALUE str, VALUE times)
  *  details of the format string.
  *
  *     "%05d" % 123                              #=> "00123"
- *     "%-5s: %08x" % [ "ID", self.object_id ]   #=> "ID   : 200e14d6"
+ *     "%-5s: %016x" % [ "ID", self.object_id ]  #=> "ID   : 00002b054ec93168"
  *     "foo = %{foo}" % { :foo => 'bar' }        #=> "foo = bar"
  */