Fix the example for bsearch

As mentioned in [the documentation for `bsearch`]
(http://www.ruby-doc.org/core-2.0/Range.html#method-i-bsearch), a
boolean-returning block passed to `bsearch` must satisfy the following
condition:

> there must be an index `i` (`0 <= i <= ary.size`) so that:
> 
> * the block returns false for any element whose index is less than `i`, and
> * the block returns true for any element whose index is greater than or equal
>   to `i`.

That (especially the second item) is not satisfied by the current code
(`i == needle`). Changing it to `i >= needle` solves the problem.