Allow big bang release PR

[secobaka]

@onk @motemen
Hello,
I have discovered that unintended behavior occurs during the big bang release typically seen in the early stages of repository setup.

Issue

Due to the checkbox state verification being based on a prefix match of the PR number, when multiple PRs are included, checkboxes may be unintentionally marked for PRs that weren't intended.

// Old Body
- [x] #3 Provides a creating release pull-request object for template @hakobe
- [ ] #6 Support two factor auth @ninjinkun

// New Body
- [ ] #3 Provides a creating release pull-request object for template @hakobe
- [ ] #6 Support two factor auth @ninjinkun
- [ ] #30 Extract logic from bin/git-pr-release @banyan

// Expected 
- [x] #3 Provides a creating release pull-request object for template @hakobe
- [ ] #6 Support two factor auth @ninjinkun
- [ ] #30 Extract logic from bin/git-pr-release @banyan

// Actual
- [x] #3 Provides a creating release pull-request object for template @hakobe
- [ ] #6 Support two factor auth @ninjinkun
- [x] #30 Extract logic from bin/git-pr-release @banyan

Solution

Modify to check for the existence of a space after the PR number.

[onk]

Thank you for the Pull Request.
How about the suggested change?