Fix Documentation Grammar

[Chadowo]

Hey there, so this PR is the continuation from #27 but based on the refactor. This time I checked all the files under mrb_doc/. So src/ruby/ is left.

Notable Changes

• Usage of the @example tag for code snippets instead of monospaced formatting. Before: After:
• Usage of braces to link classes in the docs instead of monospaced formatting.

---

Also, other change I've gone ahead and did is to format some code snippets that have code like this:

puts some_value 
# => 123

To this:

puts some_value # => 123

IMO the latter form with the inline comment is easier to read, since you don't have to go down a line to find the comment that indicates the return value of the method. What do you think?

[HellRok]

Thank you for taking the time to do this, I really appreciate it!