Fixed version of macports.plugin.zsh to fix issue #1983

I think it may be useful to put the export command for macports path in the plugin.
MacPorts adds this line when finishes its installation.
So if macports already installed, when oh-my-zsh install script runs,
the line disappear.
If macports is installed after oh-my-zsh, the added lines at the end of the .zshrc need to be removed.

This update fix issue #1983.