fix(docs): 404 on the Web analytics installation`s ruby on rails link

[jnettome]

Changes

Currently, on the Web Analytics installation guide, the Ruby on Rails SDK link (at the top of the page content) returns a 404: https://posthog.com/docs/libraries/rails

This is a very small PR that simply updates the link to the correct URL: https://posthog.com/docs/libraries/ruby-on-rails

Checklist

• I've read the docs and/or content style guides.
• Words are spelled using American English
• Use relative URLs for internal links
• I've checked the pages added or changed in the Vercel preview build
• If I moved a page, I added a redirect in vercel.json

[ivanagas]

thanks!