Fix Google Slides templates link

Why is this change being made?

At the Presentations section our templates link has an incorrect url: https://about.gitlab.com/handbook/tools-and-tips/#google-slides-templates instead of https://about.gitlab.com/handbook/tools-and-tips/#updating-your-slide-deck-theme

How will this merge request impact page speed?

No

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