Improve CONTRIBUTING.rdoc

[ybiquitous]

This fixes some trivial mistakes in the CONTRIBUTING document without changing the content largely.

• Fix typos
• Add commas for readability
• Fix grammar mistakes
• Fix broken links
• Use monospace font (+ or <tt>) where necessary

The current published page: https://ruby.github.io/rdoc/CONTRIBUTING_rdoc.html

[ybiquitous]

One failure on CI is caused by the Ruby installation. It seems like a temporary issue (re-run can recover?).