Spruce up the docs for new experiment Vue component a bit

What does this MR do?

Fixes up a couple of spots in the docs added for the new experiment Vue component.

• Be consistent in the name of our example experiment (use button_name as it better relates to the rest of the example)
• Be consistent in our use of the terms "multivariate" (multiple variables in a single experience) and "multiple variant" (multiple self-contained experiences)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it’s a docs-only change.
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Follow up from !56911 (merged)