small documentation bug related to viz_palette and viz_gradient

[matthewstern]

devtools::document() writes the file viz_palette.Rd with the line \name{viz_gradient}, which breaks some links to help files on pkgdown--particularly, at a minimum, the viz_palette() and viz_gradient() auto-created-links on the colors vignette/article. I have manually overwritten the \name line in the Rd file to viz_palette. Something in the roxygen comments must need to be tweaked so the name gets auto-generated correctly in the future. Otherwise, this fix may need to be manually re-imposed for future releases.