Fix header slack header in handbook/communication

Why is this change being made?

#### Need to add a new app to Slack was included in the "Not Public" section.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what