Correct dot product example calculation result

[rlhk]

Website change review checklist:

• [x] I have requested a review from the appropriate persons
• [x] I have checked that my changes work in the preview
• [x] I have checked that the rest of the site is still functioning in the preview
• [x] I have checked that my content/code is consistent with existing content/code
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This pull request:

• Correct dot product example calculation result

[kurtbruns]

Thank you for fixing this mistake!