Uses variable in install instructions

INSTALL.md

		export OWNER=username
		export WEBDIR=/var/www
		+ export WEBOWNER=www-data
		```
		1. Download code:
		```bash
		mkdir -p /var/www/${REPO}
		- cd /var/www/${REPO}
		+ cd ${WEBDIR}/${REPO}
		git clone https://repo.autonoma.ca/repo/treetrek
		- sudo usermod -aG ${OWNER} www-data
		+ sudo usermod -aG ${OWNER} ${WEBOWNER}
		```
		1. Edit `${WEBDIR}/${REPO}/Config.php`.

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