DOC: Fix syntax error in groupby docs
https://travis-ci.org/pandas-dev/pandas/jobs/450582832

#15559 has been rearing its head again.  Turns out the reason for the timeout is one silly, missing parenthesis in the `groupby.rst` docs (that was the cause as well in #15506). 😆 

Follow-up to #23394.
> maybe make this actually do something? e.g .sum() to show the resulting frame

@jreback : That's fair.  I do want to make sure that the [Travis build](https://travis-ci.org/pandas-dev/pandas/jobs/450606281) first doesn't timeout.  Then we can add something to show the result. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23498?src=pr&el=h1) Report
> Merging [#23498](https://codecov.io/gh/pandas-dev/pandas/pull/23498?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/24ab22f75aae1c9b9491c26e6f753814b7d6fd8e?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/23498/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23498?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23498   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23498?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/23498?src=pr&el=footer). Last update [24ab22f...5248094](https://codecov.io/gh/pandas-dev/pandas/pull/23498?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Made the suggested change, and all builds (including the doc build) are green.  PTAL. @gfyoung Thanks for finding the trigger for the doc time out! Luckily it was something very simple in the end ;) (I still don't understand how this caused such a slow down without giving actual errors) 

(but maybe in the future if you are re-doing a PR that broke the doc build, make sure to check if now the doc build is actually working .. :)) > (but maybe in the future if you are re-doing a PR that broke the doc build, make sure to check if now the doc build is actually working .. :))

@jorisvandenbossche : The doc builds were working in fact when I first opened the PR.  It was a mysterious bug that reared its head in strange ways. 🙂 @gfyoung yeah, I can only see the status of the last commit (in general, can you start adding new commits instead of rebasing/amending a single commit in a PR ?), and there it was actually failing, although it looks green. I think we discussed that before, but we should probably move the doc build out of "allowed failures".