DOC: Fix versionadded directive typos in IntervalIndex
There needs to be a space between the `..` and `versionadded::`.  Currently being rendered as plaintext, which doesn't look too great:

![image](https://user-images.githubusercontent.com/5332445/41943354-50072132-7960-11e8-814e-5fd8ef559a86.png)

Couldn't find any other instances of this in the codebase for `versionadded` or any other commonly used directives.  Pretty minor so tagging this as 0.23.2.
