[ruby/open-uri] Improve URI.open documentation with usage exampleHEADmaster

This improves the `URI.open` method documentation by adding a code example requiring `open-uri` as a basic usage.

When reading the current documentation first, I didn't realize that `open-uri` was required to call the method.
I believe the improved version could be more helpful for new users.

```sh-session
$ ruby -r uri -e 'p URI.open("http://example.com")'
-e:1:in '<main>': private method 'open' called for module URI (NoMethodError)
```

Ref https://docs.ruby-lang.org/en/master/URI.html#method-c-open

Also, this improves formatting with code fonts for better readability.

https://github.com/ruby/open-uri/commit/f4400edc27

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