Fix ruby version in Readme

[JuanVqz]

An alternative could be not to specify the version and ask them to check the .tool-version file

Fixes #2070

If you’re adding a new scraper, please ensure that you have:

• Tested the scraper on a local copy of DevDocs
• Ensured that the docs are styled similarly to other docs on DevDocs

• Added these files to the public/icons/your_scraper_name/ directory:
  • 16.png: a 16×16 pixel icon for the doc
  • [email protected]: a 32×32 pixel icon for the doc
  • SOURCE: A text file containing the URL to the page the image can be found on or the URL of the original image itself

If you're updating existing documentation to its latest version, please ensure that you have:

• Updated the versions and releases in the scraper file
• Ensured the license is up-to-date and that the documentation's entry in the array in about_tmpl.coffee matches its data in self.attribution
• Ensured the icons and the SOURCE file in public/icons/your_scraper_name/ are up-to-date if the documentation has a custom icon
• Ensured self.links contains up-to-date urls if self.links is defined
• Tested the changes locally to ensure:
  • The scraper still works without errors
  • The scraped documentation still looks consistent with the rest of DevDocs
  • The categorization of entries is still good

[ojeytonwilliams]

Thanks @JuanVqz. LGTM 👍