BIGTOP-3643: Upgrade gradle task description from ubuntu 16.04 to 18.04

[yoda-mon]

Description of PR

Upgrade the usage example on the task description, from ubuntu 16.04 to 18.04.
Bigtop 3 does not support ubuntu 16.04 so this PR would help users to avoid misunderstanding.

How was this patch tested?

Check by hand,

./gradlew tasks|grep ubuntu-16.04
./gradlew tasks|grep ubuntu-18.04

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'BIGTOP-3638. Your PR title ...')?
• Make sure that newly added files do not have any licensing issues. When in doubt refer to https://www.apache.org/licenses/

[iwasakims]

+1. Thanks, @yoda-mon.