BUG: Fix typo in melt value_name futurewarning
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Just removing the duplicated 'i' in "resultiing" of the futurewarning in melt about using `value_name` with the same name as a column the in the original dataframe.  Filing an issue or a whatsnew entry for such a minor change seemed excessive.