Fix handbook target for Women in Management

Why is this change being made?

I was reading about our GitLab Women TMRG and people KPIs, and noticed that the Women in Management target is stated as 40% and but only 33% in the embedded Sisense chart. It's also linked to as 30% on another page. I'm assuming Sisense is the most updated value here, but am not really sure of that.

Per the company KPIs page and recent People Group GC slides, I've updated this change to use our 40% target.

In any event, we should update these pages to reflect our target.

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
• Assign this change to the correct DRI
  • 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.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)