Fix double ` in 'Reshaping by Melt' section
See current stable docs for the issue: https://pandas.pydata.org/pandas-docs/stable/reshaping.html#reshaping-by-melt

The double ` is causing the entire paragraph to be fixed width until the next double `. This commit removes the extra `

 - [ ] 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/17030?src=pr&el=h1) Report
> Merging [#17030](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/031d7a9fe24f2799454de34a1e595ae4fa6cfc9f?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17030/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17030      +/-   ##
==========================================
- Coverage      91%   90.98%   -0.02%     
==========================================
  Files         161      161              
  Lines       49304    49304              
==========================================
- Hits        44867    44858       -9     
- Misses       4437     4446       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.76% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17030?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/17030?src=pr&el=footer). Last update [031d7a9...e0ba46b](https://codecov.io/gh/pandas-dev/pandas/pull/17030?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Good catch!  Thanks @nickgarvey !