[docs] GettingStarted.rst cmake should reference build

[docs] GettingStarted.rst cmake should reference build

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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D153727