fix(Guild): change wrong description & call fetchVanityData internally

[papaia]

Please describe the changes this PR makes and why it should be merged:
The docs for vanityURLUses mention the need to use Guild#fetchVanityCode (and not Guild#fetchVanityData), which is wrong.
Also, this PR makes it that even when only calling fetchVanityCode, it will internally call fetchVanityData, so the users would be set anyway.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.
  This PR includes internal code changes as well as documentation changes.