fix(ClientApplication): type fetchAssets as resolving with an array

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR fixes the return type of ClientApplication#fetchAssets in your type declarations.
It incorrectly stated that it would return a Promise<ClientApplicationAsset> instead of Promise<ClientApplicationAsset[]>.

Fixes #4026.

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.