Update api.rst
remove duplicate line for DataFrame.insert(...)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23052?src=pr&el=h1) Report
> Merging [#23052](https://codecov.io/gh/pandas-dev/pandas/pull/23052?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f4fe6e30ef0d3e51d49261f56adaecc91436adfb?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/23052/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23052?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23052   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         169      169           
  Lines       50898    50898           
=======================================
  Hits        46929    46929           
  Misses       3969     3969
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23052?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/23052?src=pr&el=footer). Last update [f4fe6e3...389ee29](https://codecov.io/gh/pandas-dev/pandas/pull/23052?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!