fix tiny typo in lib/io/file.dart

Closes #54562

GitOrigin-RevId: f3b2502
Change-Id: I6af749c16a5358e910bc883ef1ddfc65c30a0c3c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345369
Reviewed-by: Lasse Nielsen <[email protected]>
Commit-Queue: Kevin Moore <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>

Author: osaxma

sdk/lib/io/file.dart

@@ -710,7 +710,7 @@ abstract interface class RandomAccessFile {
 
   /// Reads bytes into an existing [buffer].
   ///
-  /// Reads bytes and writes then into the range of [buffer]
+  /// Reads bytes and writes them into the range of [buffer]
   /// from [start] to [end].
   /// The [start] must be non-negative and no greater than [buffer].length.
   /// If [end] is omitted, it defaults to [buffer].length.