docs: fix missing trailing periods and Title Case heading in CONTRIBUTING.md

[amir-rezaei]

Description

This PR fixes sentence trailing periods and section heading Title Case capitalization in CONTRIBUTING.md.

Details

1. Added missing trailing periods to sentence ends.
2. Capitalized section header (## ZenHub project -> ## ZenHub Project).

[tekton-robot]

@amir-rezaei: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

Details

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[tekton-robot]

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[Copilot]

Pull request overview

This PR makes small documentation style fixes in CONTRIBUTING.md by adding missing trailing periods and adjusting a section heading to Title Case.

Changes:

• Add trailing periods to a ZenHub project list item and the Pipelines roadmap sentence.
• Capitalize the “ZenHub Project” section heading.

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[vdemeester]

I think we also changed a bit where the roadmap is.. see https://github.com/tektoncd/pipeline/blob/main/roadmap.md

[anithapriyanatarajan]

please follow the PR template. Release Note section has to be populated as applicable. Complete the EasyCLA step - https://api.easycla.lfx.linuxfoundation.org/v2/repository-provider/github/sign/8049964/196359824/3807/#/?version=2