Fix typo in ubuntu_setup.md (apache#17496)

docs/static_site/src/pages/get_started/ubuntu_setup.md

@@ -132,9 +132,12 @@ the guide in [Math Library Selection](build_from_source#math-library-selection).
 rm -rf build
 mkdir -p build && cd build
 cmake -GNinja -C ../config.cmake ..
-cmake --build
+cmake --build . --parallel 8
 ```
 
+You can increase the `--parallel 8` argument to match the number of processor
+cores of your computer.
+
 After a successful build, you will find the `libmxnet.so` in the `build` folder
 in your MXNet project root. `libmxnet.so` is required to install language
 bindings described in the next section.