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.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=h1) Report
> Merging [#21649](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/45e55af9cf74aede00a8b10f8922537f285a573a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21649/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21649   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49558    49558           
=======================================
  Hits        45545    45545           
  Misses       4013     4013
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <ø> (ø)` | :arrow_up: |
| #single | `42.02% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/21649/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.16% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=footer). Last update [45e55af...2d3602a](https://codecov.io/gh/pandas-dev/pandas/pull/21649?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!