I fix ISO no in String class in comparison with ISO_IEC_30170_2012(E)

I fix ISO no in String class in comparison with ISO_IEC_30170_2012(E)-Charactor_PDF_document.pdf

ISO no:
```ruby
15.2.10.5.1 String#<=>
15.2.10.5.2 String#==
15.2.10.5.4 String#+
15.2.10.5.5 String#*
15.2.10.5.38 String#to_f
15.2.10.5.39 String#to_i
```

Not ISO:
```ruby
String#to_str
```
String#to_s is ISO spec but String#to_str is not ISO.