Update README.md

I dunno, I didn't mind how it was before.
```ruby
kind =
  case year
  when 1850..1889
    'Blues'
  when 1890..1909
    'Ragtime'
  when 1910..1929
    'New Orleans Jazz'
  when 1930..1939
    'Swing'
  when 1940..1950
    'Bebop'
  else
    'Jazz'
  end
```

Or even 

(I, personally, don't like this style too much because of the whitespace beneath `kind =` and it's slightly more annoying to line up than the former.):

```ruby
kind = case year
       when 1850..1889
         'Blues'
       when 1890..1909
         'Ragtime'
       when 1910..1929
         'New Orleans Jazz'
       when 1930..1939
         'Swing'
       when 1940..1950
         'Bebop'
       end
```

I, again personally, think it's a bit easier to read when the entire if/case statement has a shared indentation level. Mind talking a bit about the version in the PR? Also, if we're going to go with this, we probably want to remove the lines above that say this style is `# bad - pretty convoluted` :stuck_out_tongue: