update to stdout page, (#656)

runtime/tutorials/unix_cat.md

@@ -11,8 +11,8 @@ oldUrl:
  line arguments.
 - [Deno.open](https://docs.deno.com/api/deno/~/Deno.open) is used to get a
  handle to a file.
-- [Deno.stdout.writable](https://docs.deno.com/api/deno/~/Deno.stdout.writable)
- is used to get a writable stream to the console standard output.
+- [Deno.stdout.writable](https://docs.deno.com/api/deno/~/Deno.stdout) is used
+ to get a writable stream to the console standard output.
 - [Deno.FsFile.readable](https://docs.deno.com/api/deno/~/Deno.FsFile#property_readable)
  is used to get a readable stream from the file. (This readable stream closes
  the file when it is finished reading, so it is not necessary to close the file