DOC: Use backticks instead of quotes
Small formatting edit to whatsnew.  I think there should be backticks and not single quotes here?
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=h1) Report
> Merging [#24744](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/33f91d8f9f2e84f2b5f3ac3f0481b691c977c427?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/24744/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24744   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48373    48373           
  Misses       3985     3985
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `43.07% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24744?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/24744?src=pr&el=footer). Last update [33f91d8...00c542d](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=h1) Report
> Merging [#24744](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/33f91d8f9f2e84f2b5f3ac3f0481b691c977c427?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/24744/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24744   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48373    48373           
  Misses       3985     3985
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `43.07% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24744?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/24744?src=pr&el=footer). Last update [33f91d8...00c542d](https://codecov.io/gh/pandas-dev/pandas/pull/24744?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks