DOC: Fixing broken references in the docs
Just fixing some typos that are breaking references in the docs.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=h1) Report
> Merging [#24497](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5e5bf761092c59eeb9b8750f05f2bc29fb45927?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/24497/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24497   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         166      166           
  Lines       52412    52412           
=======================================
  Hits        48382    48382           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.73% <ø> (ø)` | :arrow_up: |
| #single | `43.05% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/24497/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.8% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24497?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/24497?src=pr&el=footer). Last update [d5e5bf7...cc25b35](https://codecov.io/gh/pandas-dev/pandas/pull/24497?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/24497?src=pr&el=h1) Report
> Merging [#24497](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5e5bf761092c59eeb9b8750f05f2bc29fb45927?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/24497/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24497   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         166      166           
  Lines       52412    52412           
=======================================
  Hits        48382    48382           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.73% <ø> (ø)` | :arrow_up: |
| #single | `43.05% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/24497/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.8% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24497?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/24497?src=pr&el=footer). Last update [d5e5bf7...cc25b35](https://codecov.io/gh/pandas-dev/pandas/pull/24497?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!
