readme: use apt(8) on debian

apt-get(8) has been replaced with apt(8) on Debian stable.

README.md

@@ -53,7 +53,7 @@ If you've already partially compiled servo but forgot to do this step, run `./ma
 #### On Debian-based Linuxes
 
 ``` sh
-sudo apt-get install git curl freeglut3-dev autoconf \
+sudo apt install git curl freeglut3-dev autoconf \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
     gperf g++ build-essential cmake virtualenv python-pip \
     libssl1.0-dev libbz2-dev libosmesa6-dev libxmu6 libxmu-dev \