Fix `ZoneBinaryCallback` documentation

There was a small mistake in the documentation, because it was copy-pasted from `ZoneUnaryCallback` and not fixed.

Closes https://github.com/dart-lang/sdk/pull/47693
https://github.com/dart-lang/sdk/pull/47693

GitOrigin-RevId: 49599cdf2e89d7861378e56a71f69f4e0c47b53b
Change-Id: I621e71cc316798b88ad14f048b75f4fefc427441
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/220136
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Kevin Moore <kevmoo@google.com>
diff --git a/sdk/lib/async/zone.dart b/sdk/lib/async/zone.dart
index cb63080..43e377a 100644
--- a/sdk/lib/async/zone.dart
+++ b/sdk/lib/async/zone.dart
@@ -797,7 +797,7 @@
 
   /// Registers the given callback in this zone.
   ///
-  /// Similar to [registerCallback] but with a unary callback.
+  /// Similar to [registerCallback] but with a binary callback.
   ZoneBinaryCallback<R, T1, T2> registerBinaryCallback<R, T1, T2>(
       R callback(T1 arg1, T2 arg2));