[dart:io] Update inaccurate comments of writeByteSync

Closes: #42298
Change-Id: Ide2a5318c185530368ae3ebded3c3c0fb86e8338
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151162
Reviewed-by: Jonas Termansen <[email protected]>
Commit-Queue: Zichang Guo <[email protected]>

Author: zichangg

sdk/lib/io/file.dart

@@ -726,8 +726,7 @@ abstract class RandomAccessFile {
   Future<RandomAccessFile> writeByte(int value);
 
   /**
-   * Synchronously writes a single byte to the file. Returns the
-   * number of bytes successfully written.
+   * Synchronously writes a single byte to the file. Returns 1 on success.
    *
    * Throws a [FileSystemException] if the operation fails.
    */

sdk_nnbd/lib/io/file.dart

@@ -724,8 +724,7 @@ abstract class RandomAccessFile {
   Future<RandomAccessFile> writeByte(int value);
 
   /**
-   * Synchronously writes a single byte to the file. Returns the
-   * number of bytes successfully written.
+   * Synchronously writes a single byte to the file. Returns 1 on success.
    *
    * Throws a [FileSystemException] if the operation fails.
    */