Fix fix on 2004/gavin/fs.tar

The tarball, whilst in a subdirectory, did not extract into a
subdirectory so that if one did tar xvf img/fs.tar it would still
overwrite the files. As well make clobber would wipe the files out. This
does mean one will have to update the tarball if they are to use it but
one presumes that they would know how to do this.

Note that kernel is still under img/ but it's not part of the tarball
and it's supposed to be outside a tarball too.

2004/gavin/README.md

@@ -67,7 +67,10 @@ Press and hold any key.
 [Yusuke Endoh](/winners.html#Yusuke_Endoh)  provided the `kernel` and `fs.tar`
 files which can be used if you cannot normally use this entry. Instead of
 generating the files just use the files provided, found under the [img/](img/)
-directory.
+directory. Note that the `img/fs.tar` extracts into `fs/` so you will have to
+fix the tarball; this is done to prevent extraction from the entry directory
+overwriting the files and preventing `make clobber` from wiping some of them
+out.
 
 Thank you Yusuke!