[docs] GettingStarted: Default to "hosttools" bootstrapping mode on macOS

Author: AnthonyLatsis

docs/HowToGuides/GettingStarted.md

@@ -259,7 +259,8 @@ Phew, that's a lot to digest! Now let's proceed to the actual build itself!
      ```sh
      utils/build-script --skip-build-benchmarks \
        --skip-ios --skip-watchos --skip-tvos --swift-darwin-supported-archs "$(uname -m)" \
-       --sccache --release-debuginfo --swift-disable-dead-stripping
+       --sccache --release-debuginfo --swift-disable-dead-stripping \
+       --bootstrapping=hosttools
      ```
    - Linux:
      ```sh