[SYCL][Doc] Rename private_alloca extension proposal file

[victor-eds]

private_alloca extension proposal file was misnamed. Rename to sycl_ext_oneapi_private_alloca.asciidoc.

[bader]

Post-commit failures:

Complete job name: e2e-lin (Intel Arc A-Series Graphics with Level Zero, ["Linux", "arc"], --param matrix-xmx8=True ... / Intel Arc A-Series Graphics with Level Zero

Failed Tests (2):
  SYCL :: Basic/built-ins/marray_common.cpp
  SYCL :: Basic/built-ins/marray_relational.cpp

Complete job name: e2e-win / Intel GEN12 Graphics with Level Zero

lit.py: D:\github\actions-runner\_work\llvm\llvm\llvm\sycl\test-e2e\lit.cfg.py:613: fatal: Cannot list device aspects for level_zero:gpu
Command ' d:\github\actions-runner\_work\llvm\llvm\install\bin\sycl-ls.exe --verbose' returned non-zero exit status 322122[54](https://github.com/intel/llvm/actions/runs/8113901736/job/22179902441#step:12:55)77.

Complete job name: e2e-lin (Intel GEN12 Graphics with Level Zero, ["Linux", "gen12"], --param gpu-intel-gen12=True, ... / Intel GEN12 Graphics with Level Zero

Failed Tests (2):
  SYCL :: Basic/built-ins/marray_common.cpp
  SYCL :: Basic/built-ins/marray_relational.cpp

Obviously, documentation file renaming can't cause these failures.