Ubuntu 18.04 doesn't have a new enough version of Wayland for SDL, so update documentation appropriately

Reference https://github.com/libsdl-org/SDL/issues/7271

docs/README-linux.md

@@ -17,11 +17,11 @@ Ubuntu 18.04, all available features enabled:
     sudo apt-get install build-essential git make \
     pkg-config cmake ninja-build gnome-desktop-testing libasound2-dev libpulse-dev \
     libaudio-dev libjack-dev libsndio-dev libsamplerate0-dev libx11-dev libxext-dev \
-    libxrandr-dev libxcursor-dev libxfixes-dev libxi-dev libxss-dev libwayland-dev \
+    libxrandr-dev libxcursor-dev libxfixes-dev libxi-dev libxss-dev \
     libxkbcommon-dev libdrm-dev libgbm-dev libgl1-mesa-dev libgles2-mesa-dev \
     libegl1-mesa-dev libdbus-1-dev libibus-1.0-dev libudev-dev fcitx-libs-dev
 
-Ubuntu 22.04+ can also add `libpipewire-0.3-dev libdecor-0-dev` to that command line.
+Ubuntu 22.04+ can also add `libpipewire-0.3-dev libwayland-dev libdecor-0-dev` to that command line.
 
 Fedora 35, all available features enabled: