getting-started.html: add <code> around mention of `placeholder`
getting-started.html: add <code> around mention of `placeholder`

BS3 commit 29b4caf1d5230a78bfc5f3e4d698d1b5c92e1930

2947721120/todc-bootstrap,ztx1491/todc-bootstrap,2947721120/googlebt4,todc/todc-bootstrap,aaroncnhk/todc-bootstrap,mozillo/todc-bootstrap,todc/todc-bootstrap,up2vs/todc-bootstrap,todc/todc-bootstrap