[docs] README is mis-titled

[farmanp]

resolves: #233
related: dbt-labs/dbt-core#10384

Problem

Header name was incorrect

Solution

Correct it from dbt-test-adapter to dbt-adapters

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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