fix(css): Update to $gray-10 variable

What does this MR do?

A hot fix for a missing $gray-0 variable that should be $gray-10:

ERROR in ./src/scss/storybook.scss (./node_modules/css-loader/dist/cjs.js!./node_modules/postcss-loader/src??ref--3-2!./node_modules/sass-loader/lib/loader.js??ref--3-3!./src/scss/storybook.scss)
Module build failed (from ./node_modules/sass-loader/lib/loader.js):

$card-cap-bg: $gray-0;
             ^
      Undefined variable: "$gray-0".
      in /Users/davidoregan/dev/gitlab/gitlab-ui/src/scss/variables.scss (line 462, column 15)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.