fix typo (sshnet#999)

src/Renci.SshNet/Sftp/SftpFileReader.cs

@@ -82,7 +82,7 @@ public byte[] Read()
 
             lock (_readLock)
             {
-                // wait until either the next chunk is avalable, an exception has occurred or the current
+                // wait until either the next chunk is available, an exception has occurred or the current
                 // instance is already disposed
                 while (!_queue.TryGetValue(_nextChunkIndex, out nextChunk) && _exception == null)
                 {