Fix for n‐update failing to update the installed copy of n

If you've run into problems when running n-update as described in #10, you can fix the problem by updating your already-installed copy of n-update directly:

• cd "$N_PREFIX/bin"
• curl https://raw.githubusercontent.com/mklement0/n-install/stable/bin/n-update > n-update
• and then run n-update again.

This should fix the problem persistently.