refined PR template

[StrikerRUS]

Checklist

• [ ] All new jobs, commands, executors, parameters have descriptions
• [ ] Examples have been added for any significant new features
• [ ] README has been updated, if necessary

Motivation, issues

Refer to https://github.com/CircleCI-Public/orb-starter-kit/pull/19#pullrequestreview-298343472.

Description

Updates PR templates handling according to GitHub guidelines.

[StrikerRUS]

@KyleTryon Please review.

[KyleTryon]

Thank you!