docs: replace HTML entity ' with literal apostrophe

[martijnckx]

Fix in docs

On the Plugins > Build your own page, I saw some HTML entities ' printed to the page in literal form instead of being rendered as an apostrophe.

I checked other pages, and they just use an actual apostrophe in the mdx files (on the Plugins > SEO page for example).

I also checked if ' was used on any other mdx pages, and that was not the case.

So I replaced all instances of ' with ' on the Plugins > Build your own page, which are now rendered correctly.