Replace bold with code

[WebVPF]

This pull request is to improve the usability of the documentation.
The problem is that browsers translate values in strong tags. Values in tags code are skipped by browsers during translation.

Now I see the page like this:

[WebVPF]

Should such cases be reported?

Page https://wintercms.com/docs/plugin/registration#extending-twig
If there is an opportunity to fix it, show me how I will fix it when found.

[WebVPF]

I finished.

[bennothommo]

Great work @WebVPF. If you're willing, could you have a look at other pages and make the same changes if required?