Fix typo in error message in the PlanePlot class
- [x] closes #21347 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Very simple fix, just a typo in an error message.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21350?src=pr&el=h1) Report
> Merging [#21350](https://codecov.io/gh/pandas-dev/pandas/pull/21350?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0c65c57a279e755ab7093db925d1e580f9878dae?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21350/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21350?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21350   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45512    45512           
  Misses       4037     4037
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <0%> (ø)` | :arrow_up: |
| #single | `41.87% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21350?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/21350/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.39% <0%> (ø)` | :arrow_up: |

------

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> **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/21350?src=pr&el=footer). Last update [0c65c57...aad503b](https://codecov.io/gh/pandas-dev/pandas/pull/21350?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!