FIX: typos in navbar documentation

[CalMacCQ]

Mini PR to fix two typos I found whilst reading the header and navbar documentation.

Edit: seems like the docs build is failing - will take a look (The build succeds locally)

https://sphinx-book-theme.readthedocs.io/en/stable/sections/header.html

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[agoose77]

Thanks! These kind of changes are hard to get around to, so it helps to have people like you making maintainers lives much easier!

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