build: fix a message
build: fix a message

There is no tools/npm-update.  It got renamed to npm-install during
reviews, but I missed changing this message.

Update the message accordingly.

garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit