DOC: RangeIndex as default index
Small correction to DataFrame docstring
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19781?src=pr&el=h1) Report
> Merging [#19781](https://codecov.io/gh/pandas-dev/pandas/pull/19781?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/af5e8ecc800493c0f2578daa25354ae32e08b544?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19781/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19781?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19781      +/-   ##
==========================================
- Coverage   91.61%    91.6%   -0.01%     
==========================================
  Files         150      150              
  Lines       48887    48887              
==========================================
- Hits        44786    44784       -2     
- Misses       4101     4103       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.8% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19781?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19781/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19781/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19781?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/19781?src=pr&el=footer). Last update [af5e8ec...2f085a7](https://codecov.io/gh/pandas-dev/pandas/pull/19781?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Unrelated test failures: https://travis-ci.org/maxim-lian/pandas/jobs/344592059 thanks!