Update Ubuntu version number references in push.md
Update Ubuntu version number references in push.md

Ubuntu version references were a mixture of 14.04 (in descriptions) and 20.04 (in example code). Updated description references to 20.04 to match example code.

Signed-off-by: Mike Dalton <c524c90ef803c68d254f14cb3094f2f48de190d3@github.com>

docker/cli,cpuguy83/cli,cpuguy83/cli,docker/cli,thaJeztah/cli,thaJeztah/cli