docs(prerequisites): add `--no-save` to `npm install`

To avoid the mistake fixed by the previous commit, ensure users install
the Node.js dependencies without polluting the lock file by passing
`--no-save` to the `npm install` line.

Change-Id: I10b5cc17b9001fc2e26deee02bf99ce033a949c1
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed