Update the levenshtein distance method in guides.

Copied from https://github.com/rails/rails/blob/master/railties/lib/rails/generators.rb#L258-L290.

Why I want to update? I update this because the current implementation has wrong result, if you copy this code into console:

```ruby
def distance(s1, s2)
  s = s1.unpack('U*')
  t = s2.unpack('U*')
  m = s.length
  n = t.length

  # matrix initialization
  d = []
  0.upto(m) { |i| d << [i] }
  0.upto(n) { |j| d[0][j] = j }

  # distance computation
  1.upto(m) do |i|
    1.upto(n) do |j|
      cost = s[i] == t[j] ? 0 : 1
      d[i][j] = [
        d[i-1][j] + 1,      # deletion
        d[i][j-1] + 1,      # insertion
        d[i-1][j-1] + cost, # substitution
      ].min
    end
  end

  # all done
  return d[m][n]
end
```

And type `distance 'kitten', 'sitting'`, the result is `2`, which is wrong. It should be `3`.

[This kitten and sitting example is from wikipedia](http://en.wikipedia.org/wiki/Levenshtein_distance#Example).