utilities: Add important usage info to toarxiv.sh

You have to run utilities/toarxiv.sh from a directory that has a
fully-built perfbook.pdf, along with all of the files generated by a
full build, so this commit adds that point briefly to the header comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
1 file changed