[docs] GettingStarted.rst cmake should reference build

[fzakaria]

Summary

The next sections in GettingStarted assume you are still in the root
directory llvm-project when using ninja.

Make the cmake --build command match it as well.

Note: I am a new cmake user and this confused me.

[fhahn]

llvm/docs/GettingStarted.rst

94–95

Should this also be updated to use --build build?

[fzakaria]

llvm/docs/GettingStarted.rst

94–95

Ah -- i'm not sure why i never received email for this.
Sorry I missed your feedback.

I think you are right.

[fzakaria]

Add missing build in documentation from review comment

[fzakaria]

• added missing build specified in docs

[fzakaria]

@fhahn thank you for the review and sorry for the wait.

Took me a little bit to remember how to use arcanist (it's been a while).
I was going through the GettingStarted guide and got bit again by this so it reminded me to check the review :)

[MaskRay]

Looks great!

[fzakaria]

In D153727#4502804, @MaskRay wrote:

Looks great!

ty :)

Following the https://llvm.org/docs/Phabricator.html; obligatory mention "I do not have commit acces".

[MaskRay]

llvm/docs/GettingStarted.rst

94–95

I will change make to make -C build while here.

[MaskRay]

BTW, you can build the docs with ninja docs-llvm-html. It requires -DLLVM_ENABLE_SPHINX=ON and pip3 install --user recommonmark sphinx sphinx-automodapi (newer pip may need --break-system-packages, I haven't figured it out yet)

llvm/docs/GettingStarted.rst

* ``-DLLVM_USE_LINKER=lld`` --- Link with the `lld linker`_, assuming it
        if the default linker is slow.
 
      * ``-DLLVM_PARALLEL_{COMPILE,LINK}_JOBS=N`` --- Limit the number of
        compile/link jobs running in parallel at the same time. This is
        especially important for linking since linking can use lots of memory. If
        you run into memory issues building LLVM, try setting this to limit the
        maximum number of compile/link jobs running at the same time.
 
-   * ``cmake --build . [--target <target>]`` or the build system specified
+   * ``cmake --build build [--target <target>]`` or the build system specified
      above directly.
 
-     * The default target (i.e. ``cmake --build .`` or ``make``) will build all of
-       LLVM.
+     * The default target (i.e. ``cmake --build build`` or ``make -C build``)
+       will build all of LLVM.
 
      * The ``check-all`` target (i.e. ``ninja check-all``) will run the
        regression tests to ensure everything is in working order.
 
      * CMake will generate build targets for each tool and library, and most
        LLVM sub-projects generate their own ``check-<project>`` target.
 
      * Running a serial build will be **slow**.  To improve speed, try running a