DOC: correct header line in v0.24.0
Small change, but I *think* that this was the cause of a bunch of warnings in the doc build
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21218?src=pr&el=h1) Report
> Merging [#21218](https://codecov.io/gh/pandas-dev/pandas/pull/21218?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6abb6148c8dc14e6279f31f2e620ecf52822107?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/21218/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21218?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21218   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <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/21218?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/21218?src=pr&el=footer). Last update [f6abb61...aa0622c](https://codecov.io/gh/pandas-dev/pandas/pull/21218?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
