DOC: Fix a typo in travis.yml
Thanks # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15915?src=pr&el=h1) Report
> Merging [#15915](https://codecov.io/gh/pandas-dev/pandas/pull/15915?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ba30e3a2e376035549b009079d44ba5ca7a4c48f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15915/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/15915?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15915      +/-   ##
==========================================
+ Coverage   90.95%   90.96%   +<.01%     
==========================================
  Files         145      145              
  Lines       49534    49534              
==========================================
+ Hits        45056    45057       +1     
+ Misses       4478     4477       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.72% <ø> (ø)` | :arrow_up: |
| #single | `40.63% <ø> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15915?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15915?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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