Remark the abstract property of Ping

[u-m-i]

Hi there! I stumble across your package in NPM, it looked great so I wanted to propose some changes; quoting from the commit:

The title for the section of the Ping class was updated to 'Abstract Class', with the intention of noting that the class is not usable unless extended. All the links to the section were updated and some code blocks where labeled with the language (cmd bash, text). A simple addition was added to the clause "Ping is an EventEmitter"

PS: Please use a squeeze if you admit the PR, I tried to amend the commit but I did it wrong, resulting a duplicate commit but with the correction in the commit message.

[u-m-i]

This is my source for modifying the code blocks