Quote title with a colon in slack-training (people-group/learning-and-development)

Why is this change being made?

cd source/handbook && NO_CONTRACTS=true bundle exec middleman --daemon --port=4567 shows the following warning (or error):

YAML Exception parsing /workspace/www-gitlab-com/sites/handbook/source/handbook/people-group/learning-and-development/learning-initiatives/slack-training/index.html.md: (<unknown>): mapping values are not allowed in this context at line 2 column 29

Follows up !106104 (merged)

Target URL: https://about.gitlab.com/handbook/people-group/learning-and-development/learning-initiatives/slack-training/

Author Checklist

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