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>
```

1 files changed, 1 insertions, 1 deletions

diff --git a/string.c b/string.c
index 83fbf285c0..254e0bdefa 100644
--- a/string.c
+++ b/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+.