Updated the prefix for the main Slack window

[alexrepty]

Slack Version 4.3.3 (and possibly before) uses a pipe instead of a dash for the main window, so this commit addresses that.

Changes this:

on «event SLAKMNWN»
	chooseWindow("Slack -")
end «event SLAKMNWN»

to this:

on «event SLAKMNWN»
	chooseWindow("Slack | ")
end «event SLAKMNWN»

Since that appears to be what Slack uses nowadays:

[alexrepty]

@samknight Sorry, totally forgot to ping you about this. This change is needed to get the AppleScript definitions working again with a current version of Sketch, since the limiter changed. Would you like to review and merge?

[samknight]

Apologies. I didn't didn't get alerted to the PR. I'm going to test this out now.

Thanks for updating!