Fixed descriptions of String#==

This describes what the code actually does. The old
version referenced a class check and the spaceship operator,
both of which aren't used.

Also fixed formatting of the whole comment to be one space from
the * as per the style guide

I must admit that I do not fully follow the C code, and when reading it I would expect it to call === on obj, instead of ==, but this is the behaviour I get, so I think I misinterpreted the C code there.

Small demonstration of current behaviour:

```
class FakeString
  def initialize string
    @string = string
  end

  def to_str
    'i must exist, but i am not actually used'
  end

  def ==(other)
    @string == other
  end
end


fake = FakeString.new('a')

puts 'a' == fake # true
puts fake == 'a' # true
puts 'b' == fake # false
puts fake == 'b' #false
```