Add example of emojiID to MessageReactionAdd

[ceriath]

Since i ran into the same issue today, the documentation for MessageReactionAdd should be improved, thanks to #750

[CarsonHoffman]

I definitely think that the documentation could be improved here, and though an example is surely helpful, I think actually defining the format would be helpful as well—it might not be immediately clear that hello:1234567654321 refers to name:id.

[adityaxdiwakar]

I might be wrong here but doesn't the format also require a colon, such as :name:id?

[CarsonHoffman]

The leading colon is necessary when specifying an emoji in the content of a message; it is not necessary when specifying an emoji in an API context. (I believe it won't fail as it will be considered part of the name, as the API seems to parse name:id from right-to-left starting from the ID and considering everything before the right-most colon as part of the name)

[FedorLap2006]

Sorry response took a long time.

[ceriath]

I added the suggestion

[FedorLap2006]

Thanks for your contribution!