fix: remove incorrect required schema key on ref data source

[youcandanch]

In #1084, I introduced a data source for refs, but it looks like the merged version is off an older commit I had pushed, which includes a required schema key on the data source that isn't used by the ref API. This PR introduces the correct code (along with a quick typo fix on the documentation).

Test output:

--- PASS: TestAccGithubRefDataSource (24.72s)
    --- PASS: TestAccGithubRefDataSource/queries_an_existing_branch_ref_without_error (12.40s)
        --- SKIP: TestAccGithubRefDataSource/queries_an_existing_branch_ref_without_error/with_an_anonymous_account (0.00s)
        --- PASS: TestAccGithubRefDataSource/queries_an_existing_branch_ref_without_error/with_an_individual_account (6.42s)
        --- PASS: TestAccGithubRefDataSource/queries_an_existing_branch_ref_without_error/with_an_organization_account (5.98s)
    --- PASS: TestAccGithubRefDataSource/queries_an_invalid_ref_without_error (12.31s)
        --- SKIP: TestAccGithubRefDataSource/queries_an_invalid_ref_without_error/with_an_anonymous_account (0.00s)
        --- PASS: TestAccGithubRefDataSource/queries_an_invalid_ref_without_error/with_an_individual_account (6.28s)
        --- PASS: TestAccGithubRefDataSource/queries_an_invalid_ref_without_error/with_an_organization_account (6.03s)
PASS
ok      github.com/integrations/terraform-provider-github/v4/github     24.745s

[youcandanch]

@kfcampbell I've got some time this week to work on the prerelease stuff but would love to have this on the branch I'm working on, let me know if there's anything I can clarify on this one!

[kfcampbell]

@youcandanch Sorry for the delay, work has been crazy the past few weeks. I'll have this merged in and released shortly; thank you for the update!