Correct latex formatting in elbow legend

[rebeccabilbro]

This PR corrects the latex formatting in the legend of the KElbowVisualizer to close #1058, and also turns frameon=True for the legend to make it a bit easier to see over the gridlines and plot.

Sample Plot

Formerly:

With this change:

CHECKLIST

• Is the commit message formatted correctly?
• Have you noted the new functionality/bugfix in the release notes of the next release?
• Included a sample plot to visually illustrate your changes?
• Do all of your functions and methods have docstrings?
• Have you added/updated unit tests where appropriate?
• Have you updated the baseline images if necessary?
• Have you run the unit tests using pytest?
• Is your code style correct (are you using PEP8, pyflakes)?
• Have you documented your new feature/functionality in the docs?
• Have you built the docs using make html?

[lwgray]

Great latex-foo skills

[lwgray]

Thanks for the quick fix!