Fix call-seq [ci skip]

`encoding` can be not only an encoding name, but also an Encoding object.

```
s = String.new('foo', encoding: Encoding::US_ASCII)
s.encoding # => #<Encoding:US-ASCII>
```

string.c

@@ -1567,7 +1567,7 @@ rb_str_resurrect(VALUE str)
 /*
  *  call-seq:
  *    String.new(string = '') -> new_string
- *    String.new(string = '', encoding: encoding _name) -> new_string
+ *    String.new(string = '', encoding: encoding) -> new_string
  *    String.new(string = '', capacity: size) -> new_string
  *
  *  Returns a new \String that is a copy of +string+.