Fix variable name in Linux build instructions

0b41a1b (CMake: expose BUILD_TESTNG option, rename LOGGER to
ENABLE_LOGGER, 2024-04-29) had originally introduced this change. It
looks like d132a1c (Merge pull request #251 from
winterheart/pstring-unittests, 2024-05-01) unintentionally removed this
change.

README.md

@@ -53,7 +53,7 @@ cmake --build --preset mac --config [Debug|Release]
 sudo dpkg --add-architecture i386
 sudo apt update
 sudo apt install -y --no-install-recommends ninja-build cmake g++ libsdl1.2-dev libsdl-image1.2-dev libncurses-dev libxext6:i386 zlib1g-dev spdlog-dev
-cmake --preset linux -D LOGGER=[ON|OFF]
+cmake --preset linux -D ENABLE_LOGGER=[ON|OFF]
 cmake --build --preset linux --config [Debug|Release]
 ```
 
@@ -62,7 +62,7 @@ cmake --build --preset linux --config [Debug|Release]
 sudo dnf update --refresh
 sudo dnf install -y ninja-build cmake gcc-c++ SDL-devel SDL_image-devel ncurses-devel libXext.x86_64 zlib-devel spdlog-devel
 sudo dnf install -y libXext.i686
-cmake --preset linux -D LOGGER=[ON|OFF]
+cmake --preset linux -D ENABLE_LOGGER=[ON|OFF]
 cmake --build --preset linux --config [Debug|Release]
 ```