[Doc] Replace PYTHON_EXECUTABLE with Python3_EXECUTABLE

[eopXD]

Summary

As topic, the variable to specify the python executable now should be this.
This is probably something that was left out in D78762.

[JDevlieghere]

Yup, this looks like an oversight. Thanks for fixing this. LGTM

llvm/docs/GettingStarted.rst

 | CMAKE_BUILD_TYPE        | Tells ``cmake`` what type of build you are trying  |
 |                         | to generate files for. Valid options are Debug,    |
 |                         | Release, RelWithDebInfo, and MinSizeRel. Default   |
 |                         | is Debug.                                          |
 +-------------------------+----------------------------------------------------+
 | CMAKE_INSTALL_PREFIX    | Specifies the install directory to target when     |
 |                         | running the install action of the build files.     |
 +-------------------------+----------------------------------------------------+
-| PYTHON_EXECUTABLE       | Forces CMake to use a specific Python version by   |
+| Python3_EXECUTABLE      | Forces CMake to use a specific Python version by   |
 |                         | passing a path to a Python interpreter. By default |
 |                         | the Python version of the interpreter in your PATH |
 |                         | is used.                                           |
 +-------------------------+----------------------------------------------------+
 | LLVM_TARGETS_TO_BUILD   | A semicolon delimited list controlling which       |
 |                         | targets will be built and linked into llvm.        |
 |                         | The default list is defined as                     |
 |                         | ``LLVM_ALL_TARGETS``, and can be set to include    |