[DOC] Fix a typo in `Integer#chr` example

The current example raises the following error.

```ruby
0..chr # => undefined local variable or method `chr' for main:Object (NameError)
```

This PR updates the example to produce the expected behavior.

```ruby
0.chr # => "\x00"
```

numeric.c

@@ -3775,7 +3775,7 @@ rb_enc_uint_chr(unsigned int code, rb_encoding *enc)
  *  represented by the value of +self+, according to the given +encoding+.
  *
  *    65.chr                   # => "A"
- *    0..chr                   # => "\x00"
+ *    0.chr                    # => "\x00"
  *    255.chr                  # => "\xFF"
  *    string = 255.chr(Encoding::UTF_8)
  *    string.encoding          # => Encoding::UTF_8