update description of PullRequestCreateCommand

|Q            |A  |
|---          |---|
|Bug Fix?     |yes|
|New Feature? |no |
|BC Breaks?   |no |
|Deprecations?|no |
|Tests Pass?  |no |
|Fixed Tickets|   |
|License      |MIT|

Add some information on using the custom template
and use the correct markup for options.

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