small problem in README.md in examples

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

In the Examples, I was following the recipe on [Find Pre-Installed Open3D Package in CMake](https://github.com/isl-org/Open3D/blob/main/examples/cmake/open3d-cmake-find-package/README.md). It all worked (yay!) but there was a minor speed-bump. In _Step 2_ for the _On Ubuntu/macOS_ case, it says to copy the sample draw app from the local Open3D repository by:

```
cp -ar Open3D/examples/cmake/open3d-cmake-find-package .
```

On my macOS (14.6.1) laptop, the command did not run, saying: **cp: the -R and -r options may not be specified together**

First of all I did not specify the `-R` and `-r` options together, but that must be a macOS `cp` problem. On the other hand it is not immediately clear why I needed _any_ of `-a`, `-r`, or `-R`. There are no subdirectories, so `-r` recursion is not an issue, and the files are normal source code text files, so `-a` preservation of file attributes similarly seems a non-issue.

Perhaps this is meant to be a more general template that would work in more complicated situations? Perhaps it has to do with differences in the cp command on Linux and macOS? (See e.g. [this](https://hostman.com/tutorials/linux-cp-command/) versus [that](https://ss64.com/mac/cp.html).) Or maybe it is just an error/typo? In any case it worked OK for me to run without `-R`/`-r`:

```
cp -a Open3D/examples/cmake/open3d-cmake-find-package .
```

Although from the second doc above, it seems like the macOS command has no `-a` option. I would be happy to fix this `README.md` file as my first foray into pull requests in Open3D, except that I need some feedback on what it should be changed to. Unless I'm missing something, it would simplify the example to remove `-ar`.