Remove `-j` option from build commands in `test/README.chroot.md`.

The build system already sets a default value for the number of
parallel building tasks (based on the number of CPU cores); trust it
instead of suggesting an arbitrary value.

Test: Render `README.chroot.md` as HTML and check the output manually
Bug: 34729697
Change-Id: I48e4f07a70dcac10db7132568abea4bf65600450
1 file changed