preface:Ubuntu: Recommend using apt instead of apt-get

The 'apt' binary has appeared in apt 1.0 back in 2014 and has been
ever since the recommended tool to install packages on Debian based
systems. It makes little sense to recommend using 'apt-get' instead
of 'apt' when even the current Debian oldstable - jessie - has apt
>=1.0.9.8 and the Ubuntu 14.04 LTS - trusty - has apt
>=1.0.1ubuntu1.

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>

Author: eddyp

src/preface.md

@@ -94,20 +94,20 @@ $ brew install qemu
 
 ``` console
 $ # arm-none-eabi-gdb
-$ sudo apt-get install gdb-arm-none-eabi
+$ sudo apt install gdb-arm-none-eabi
 
 $ # QEMU
-$ sudo apt-get install qemu-system-arm
+$ sudo apt install qemu-system-arm
 ```
 
 #### Ubuntu 18.04
 
 ``` console
 $ # gdb-multiarch -- use `gdb-multiarch` when you wish to invoke gdb
-$ sudo apt-get install gdb-multiarch
+$ sudo apt install gdb-multiarch
 
 $ # QEMU
-$ sudo apt-get install qemu-system-arm
+$ sudo apt install qemu-system-arm
 ```
 
 #### Windows