Adjusted Vagrant readme path, no whitespace in path allowed

docs/Developer-Guide_Build-Preparation.md

@@ -2,10 +2,11 @@
 
 - x86/x64 machine running any OS; 4G ram, SSD, quad core (recommended),
 - [VirtualBox](https://www.virtualbox.org/wiki/Downloads) or similar virtualization software **(highly recommended with a minimum of 20GB hard disk space for the virtual disk image)**,
-- Setting up VirtualBox and compile environment is easy following our [Vagrant tutorial](https://github.com/armbian/build/blob/master/README-Vagrant.md),
+- Setting up VirtualBox and compile environment is easy following our [Vagrant tutorial](https://docs.armbian.com/Developer-Guide_Using-Vagrant/),
 - when you don't want to build whole OS images (`KERNEL_ONLY=yes`) then [Docker](https://github.com/igorpecovnik/lib/pull/255#issuecomment-205045273), [systemd-nspawn](https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html) or other containerization software can be used,
 - **Only supported** compilation environment is [Ubuntu Xenial 16.04 x64](http://archive.ubuntu.com/ubuntu/dists/xenial-updates/main/installer-amd64/current/images/netboot/mini.iso) (**no other releases are supported**! It has to be exactly 16.04 otherwise default compiler versions might not match so if you're on an older Ubuntu release upgrade to 16.04 now, if you use a newer Ubuntu version start with 16.04 from scratch),
 - installed basic system, OpenSSH and Samba (optional),
+- no spaces in full path to the build script location allowed,
 - superuser rights (configured `sudo` or root shell).
 
 Please note that system requirements (both hardware and OS/software) may differ depending on the build environment (Vagrant, Docker, Virtualbox, native).