Update the number one goal (simple typo)

Why is this change being made?

I believe I have spotted an error in the Mission page. The number one goal says that there are three ways in which people can contribute, and that they can be seen "above". However, those three goals are defined in the text below this statement.

When I read the number one goal, I was confused about which three things were being talked about. This update may help remove that confusion.

Further updates (or rejection!) welcome. Keep up the good work, I love this handbook.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• (N/A) If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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