fix "throttle" docs typo (dart-lang/stream_transform#193)

pkgs/stream_transform/lib/src/rate_limit.dart

diff --git a/pkgs/stream_transform/lib/src/rate_limit.dart b/pkgs/stream_transform/lib/src/rate_limit.dart
index 9be0be6..299c230 100644
--- a/pkgs/stream_transform/lib/src/rate_limit.dart
+++ b/pkgs/stream_transform/lib/src/rate_limit.dart
@@ -105,7 +105,7 @@
   ///
   /// For example:
   ///
-  ///     source.throtte(Duration(seconds: 6));
+  ///     source.throttle(Duration(seconds: 6));
   ///
   ///     source: 1-2-3---4-5-6---7-8-|
   ///     result: 1-------4-------7---|