fix wrong parameter in cmd in readme-sycl.md

[arthw]

The build cmd in readme-sycl.md is wrong.
"cmake --build .." is incorrect after verify.
Correct it.

[abhilash1910]

Thanks for fix!

[limitedAtonement]

This line should not start with #.

[NeoZhangJianyu]

FP16 is not recommended. It has worse performance in most of case than fp32.
so, mark by #

[limitedAtonement]

I see, excellent observation. That is not a good way to notate that, though. I think the # should be removed and add to the comment above something like: . Not generally recommended.

[NeoZhangJianyu]

In fact, this cmd is copied from the example/sycl/build.sh at beginning.
But next editor don't know it and modify it again and again. :)

[limitedAtonement]

When running cmake --build .. -D..., I get

Unknown argument -DLLAMA_SCL=ON

Using the instructions this pull request creates works as expected. (cmake --build . not cmake --build .. is correct in the pull request.)

[NeoZhangJianyu]

When running cmake --build .. -D..., I get

Unknown argument -DLLAMA_SCL=ON

Using the instructions this pull request creates works as expected. (cmake --build . not cmake --build .. is correct in the pull request.)

This method should work well too.
But this cmd will set and build all tools in llama.cpp. it will take more time.

In latest version, there are two cmake cmds:

cmake .. -DLLAMA_SYCL=ON -DCMAKE_C_COMPILER=icx -DCMAKE_CXX_COMPILER=icpx
cmake --build . --config Release -j -v
``
The second cmd allow user choose to build one or all tools.
It will save more time.

So, I think current two cmds are better.