Fix the assertion receiver in the testIn README example

[dajiaohuang]

Correct the receiver in the README's testIn example: assertEquals is the
Kotlin test assertion, while awaitItem is the ReceiveTurbine member.

This addresses the remaining documentation typo reported in
#234 (comment). It does
not change runtime behavior or claim to reopen/resolve that closed runtime
issue. The change is one README line.

Validation on the current trunk baseline:

• jvmTest: 144 tests passed, no failures/errors/skips.
• checkKotlinAbi dokkaGenerate: passed.
• Isolated snippet tests: 3 passed, including the corrected complete README
  snippet. Its failure contains a TurbineAssertionError cause with the expected
  unconsumed Item(two) and Complete events (the current coroutine version
  wraps this in a CompletionHandlerException).
• git diff --check: passed.

Apple/mobile execution and publication were not run on this Windows host.

---

• CHANGELOG.md's "Unreleased" section has been updated, if applicable.
  Not applicable: documentation-only correction with no library behavior change.

[JakeWharton]

Next time skip the AI slop description for a one-line, obvious doc change.