lib/open3.rb: fix code formatting in documentation

Docs currently lose information when parsing `Open3` comments (notice the `opts` hash with missing key [here](http://www.rubydoc.info/stdlib/open3/Open3.capture2e) and [here](http://www.ruby-doc.org/stdlib-2.1.3/libdoc/open3/rdoc/Open3.html#method-c-capture2e)). This commit wraps them in `<code>` tags, b/c that's what I saw used [elsewhere](https://github.com/ruby/ruby/blob/512c0364b3f526bef5bed1b224fbe6bb7f253eff/io.c#L9584)

![docs-update](https://cloud.githubusercontent.com/assets/77495/4784000/89fcbad8-5d3e-11e4-9a4e-7c96ad95f46a.png)

I'd do more, but couldn't figure out how docs officially get built (there's a number of tools), or if there was a documentation standard that I could use to guide my efforts (e.g. there appears to be cross-linking, but I don't really understand how the doc tool figures out what to apply it to, or when it should/shouldn't be used). A documentation standard (or even a paragraph of prose listing the tool/command and stream-of-consciousness thoughts) would make it a lot easier to contribute here, because more work went into inferring the right thing to do than into making the changes. If one does exist, maybe link it in README.* or CONTRIBUTING.md or https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute