Fix typo in get_dataloader

[kaixi-wang]

What changes are proposed in this pull request?

Replace == with = so id key becomes _id

How is this patch tested? If it is not, please explain why.

(Details)

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

(Details in 1-2 sentences. You can just refer to another PR with a description
if this PR is part of a larger change.)

What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

[benjaminpkane]

LGTM! Good catch