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
