DOC: some cleanup of various whatsnew files
Over time, various whatsnew text files have had some errors that caused parsing errors for sphinx/rst.

This PR cleans up various such bugs that have been causing wrong parsing output.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21021?src=pr&el=h1) Report
> Merging [#21021](https://codecov.io/gh/pandas-dev/pandas/pull/21021?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a5c02d5c4225a90cda8ed7f328306bd9754d0f93?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/21021/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21021?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21021   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files         153      153           
  Lines       49502    49502           
=======================================
  Hits        45457    45457           
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <ø> (ø)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21021?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/21021?src=pr&el=footer). Last update [a5c02d5...0f60696](https://codecov.io/gh/pandas-dev/pandas/pull/21021?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 