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

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

Differential Revision: https://reviews.llvm.org/D150012

libc/docs/gpu/using.rst

@@ -17,7 +17,7 @@ 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:
 
@@ -31,7 +31,7 @@ typical ``cmake`` configuration will look like this:
      -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