CLN GH23204 remove "not" typo in assertion
- [x] closes #23204 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Remove an unwanted "not" in the assertion message.

This is my first pandas PR! Let me know if there's anything else I need to do. Thanks!
Hello @rdmontgomery! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/reshape/concat.py`](https://github.com/rdmontgomery/pandas/blob/eb0ff3f7d283da03418c4d622b826a0b9ec2a114/pandas/core/reshape/concat.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23221?src=pr&el=h1) Report
> Merging [#23221](https://codecov.io/gh/pandas-dev/pandas/pull/23221?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6030452c63189d77bfdcbf4156f4c2081fdb211e?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/23221/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23221?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23221   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50962    50962           
=======================================
  Hits        46984    46984           
  Misses       3978     3978
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.26% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23221?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23221/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.6% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23221?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/23221?src=pr&el=footer). Last update [6030452...eb0ff3f](https://codecov.io/gh/pandas-dev/pandas/pull/23221?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!