[libc][docs] Fix incorrect CMake argument in GPU documentation

[jhuber6]

Summary

Summary;
This was changed a long time ago to drop the LLVM_ prefix.

libc/docs/gpu/using.rst

 Building the GPU library
 ========================
 
 LLVM's libc GPU support *must* be built with an up-to-date ``clang`` compiler
 due to heavy reliance on ``clang``'s GPU support. This can be done automatically
 using the ``LLVM_ENABLE_RUNTIMES=libc`` option. To enable libc for the GPU,
 enable the ``LIBC_GPU_BUILD`` option. By default, ``libcgpu.a`` will be built
 using every supported GPU architecture. To restrict the number of architectures
-build, either set ``LLVM_LIBC_GPU_ARCHITECTURES`` to the list of desired
+build, either set ``LIBC_GPU_ARCHITECTURES`` to the list of desired
 architectures manually or use ``native`` to detect the GPUs on your system. A
 typical ``cmake`` configuration will look like this:
 
 .. code-block:: sh
 
   $> cd llvm-project  # The llvm-project checkout
   $> mkdir build
   $> cd build
   $> cmake ../llvm -G Ninja                                \
      -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt"        \
      -DLLVM_ENABLE_RUNTIMES="libc;openmp"                  \
      -DCMAKE_BUILD_TYPE=<Debug|Release>   \ # Select build type
      -DLIBC_GPU_BUILD=ON                  \ # Build in GPU mode
-     -DLLVM_LIBC_GPU_ARCHITECTURES=all    \ # Build all supported architectures
+     -DLIBC_GPU_ARCHITECTURES=all         \ # Build all supported architectures
      -DCMAKE_INSTALL_PREFIX=<PATH>        \ # Where 'libcgpu.a' will live
   $> ninja install
 
 Since we want to include ``clang``, ``lld`` and ``compiler-rt`` in our
 toolchain, we list them in ``LLVM_ENABLE_PROJECTS``. To ensure ``libc`` is built
 using a compatible compiler and to support ``openmp`` offloading, we list them
 in ``LLVM_ENABLE_RUNTIMES`` to build them after the enabled projects using the
 newly built compiler. ``CMAKE_INSTALL_PREFIX`` specifies the installation