Fix a typo in the documentation updating manifold.rst

[aldermartinez]

Fixing a typo in the documentation when importing manifold_embedding using the quick method

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?

[bbengfort]

@aldermartinez thanks for catching this! It's minor, but if one of our users is trying to learn Yellowbrick and the import doesn't work, they won't get very far!

[bbengfort]

@DistrictDataLabs/team-oz-maintainers the tests aren't passing here, but @rebeccabilbro fixed them in #1081