dox: Fix minor spell error reported by CI

[dNechita]

Error message: "Error: ./mainpage.dox:231: re-used ==> reused"

PR Description

Fix minor spelling error found by CI.

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[rgetz]

There is a typo in the commit message - it is doc, (short for documentation) not dox.

[dNechita]

There is a typo in the commit message - it is doc, (short for documentation) not dox.

Actually, I took a look at the last 12 commits that had an impact on the mainpage.dox file to get a sense of the commit title style but there weren't 2 styles alike. I narrowed things down to having 'doc' or 'dox' as a prefix. I picked 'dox' thinking it's more specific, giving a hint that the commit modifies doxygen files.