Add missing prerelease branch target.

[JAForbes]

Addresses this issue JAForbes/pr-release#345 (raised by @pygy)

Description

Without specifying target:master the prerelease version calculation would be incorrect as it would never find the open release candidate PR.

Motivation and Context

JAForbes/pr-release#345

How Has This Been Tested?

Reproduced and repaired in the debugger locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change
• Operations change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/changelog.md