Documentation: Serenity requires GCC 9 or higher

Gcc 8.3.0 (which is the current version in debian 10 stable) seems to
fail at building AK. New people might get stuck when they try to run
make inside the ./Build folder and fail at building serenity.

Documentation/BuildInstructions.md

@@ -30,7 +30,7 @@ sudo pacman -S --needed base-devel cmake curl mpfr libmpc gmp e2fsprogs qemu qem
 apt-get install curl cmake libmpc-devel gmp-devel e2fsprogs libmpfr-devel patch gcc
 ```
 
-Ensure your gcc version is >= 8 with `gcc --version`. Otherwise, install it (on Ubuntu) with:
+Ensure your gcc version is >= 9 with `gcc --version`. Otherwise, install it (on Ubuntu) with:
 ```bash
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt-get install gcc-9 g++-9