DOC: replaced 'Good as first PR' with 'good first issue'
- [NA] closes #xxxx
- [NA] tests added / passed
- [NA] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [NA] whatsnew entry

Was browsing https://github.com/pandas-dev/pandas/tree/master/doc and it seemed the 'Good as first PR' had no open issues, whereas 'good first issue'  has quite a few open issues.
I'm guessing the 'Good as first PR' label was not well used/got renamed to 'good first issue' label?

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18806?src=pr&el=h1) Report
> Merging [#18806](https://codecov.io/gh/pandas-dev/pandas/pull/18806?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b5f1e716d89487423bb12d6cc4e6da2b39184531?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18806/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18806?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18806      +/-   ##
==========================================
- Coverage   91.64%    91.6%   -0.05%     
==========================================
  Files         154      154              
  Lines       51430    51430              
==========================================
- Hits        47131    47110      -21     
- Misses       4299     4320      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.46% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.83% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18806?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18806/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18806/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18806/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18806?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/18806?src=pr&el=footer). Last update [b5f1e71...0779857](https://codecov.io/gh/pandas-dev/pandas/pull/18806?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @raybellwaves