Fixed typo in doc string

Author: Rick053

lib/pedantic.dart

@@ -6,7 +6,7 @@ import 'dart:async' show Future;
 
 /// Indicates to tools that [future] is intentionally not `await`-ed.
 ///
-/// In an `async` context, it is normally expected than all [Future]s are
+/// In an `async` context, it is normally expected that all [Future]s are
 /// awaited, and that is the basis of the lint `unawaited_futures`. However,
 /// there are times where one or more futures are intentionally not awaited.
 /// This function may be used to ignore a particular future. It silences the