Fix description of MessageCfg.ParseMode

[artemnikitin]

It seems that Telegram changed their API a little bit and both old options currently return an error. New options are correct one.
Details: https://core.telegram.org/bots/api

[coveralls]

Coverage decreased (-0.3%) to 49.292% when pulling a077632 on artemnikitin:master into fc8ddae on bot-api:master.

[m0sth8]

@artemnikitin Thank you for PR.

This description is about constant names. Actual values are here https://github.com/bot-api/telegram/blob/master/constants.go#L79.

// Constant values for ParseMode in MessageCfg.
const (
    MarkdownMode = "Markdown"
    HTMLMode     = "HTML"
)

[artemnikitin]

Oh, I see. Unfortunately, I didn't figure it out. I will close this PR then.