README.md: Remove wrong brackets

[davidebeatrici]

Fixes the following:

test.v:8:9: error: undefined ident: `chat_id`
    6 |     bot := vgram.new_bot('<redacted>')
    7 |     bot.send_message({
    8 |         chat_id: "<redacted>",
      |         ~~~~~~~
    9 |         text: 'yo! Made using vgram!'
   10 |     })
test.v:9:9: error: undefined ident: `text`
    7 |     bot.send_message({
    8 |         chat_id: "<redacted>",
    9 |         text: 'yo! Made using vgram!'
      |         ~~~~
   10 |     })
   11 | }
test.v:7:22: error: cannot use `map[void]string` as `vgram.ConfigSendMessage` in argument 1 to `dariotarantini.vgram.Bot.send_message`
    5 | fn main(){
    6 |     bot := vgram.new_bot('<redacted>')
    7 |     bot.send_message({
      |                      ^
    8 |         chat_id: "<redacted>",
    9 |         text: 'yo! Made using vgram!'