Small bug in doc when tagging channel

[jacklu97]

Hi all!

While playing around with a bot I'm creating I noticed that the tag @ channel (adding a space because it matches a username) used in docs as an example wouldn't actually tag the channel. This is a minor change in docs, just updating

slack-github-action/docs/sending-techniques/sending-data-slack-api-method/sending-data-slack-api-method.md

Line 38 in f234e85

text: "howdy <@channel>!"

to

text: "howdy <!channel>!"

[mwbrooks]

Hey @jacklu97, thanks a bunch for the bug report! 🙇🏻

I've put together PR #447 to update the docs and gave you a shoutout! 🎩