Fix links to issues in the contributing guidelines

[MisRob]

Summary

Links need to be updated from the obsolete "TAG: help wanted" and "TAG: good first issue" to "help wanted" and "good first issue" to correspond to the latest updates of the two GitHub labels (motivated by GitHub promotion of "good first issue" label in this literal format)

Reviewer guidance

Links should take you to correct search results

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b3.dev0_git.5.g0c469b5a.pex
Windows Installer (EXE)	kolibri-0.16.0b3.dev0+git.5.g0c469b5a-unsigned.exe
Debian Package	kolibri_0.16.0b3.dev0+git.5.g0c469b5a-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b3.dev0+git.5.g0c469b5a-0.3.0.dmg
TAR file	kolibri-0.16.0b3.dev0+git.5.g0c469b5a.tar.gz
WHL file	kolibri-0.16.0b3.dev0+git.5.g0c469b5a-py2.py3-none-any.whl