howto/build-run: Bump recommended Node version to 10.
howto/build-run: Bump recommended Node version to 10.

The latest LTS is now 10.x.  I've been running 10.15.1 for a
few weeks now, with no trouble.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile