Fix MCP concern syntax hint

[apiraino]

Fixing a typo when suggesting the syntax to raise concerns on compiler MCPs (template was implemented in #1747)

[ehuss]

Does rfcbot just ignore these commands because it is not enabled on the repo?

What happens if you do @rustbot concern on it?

[apiraino]

Does rfcbot just ignore these commands because it is not enabled on the repo?

What happens if you do @rustbot concern on it?

Basically a no-op (see comment)

[ehuss]

I'm confused. The link to that comment uses @rustbot. Nothing in that PR seems to mention rfcbot.
Does rfcbot also ignore it?
Do you want to change this just to be more consistent with the use of rfcbot in other repositories?

[apiraino]

@rfcbot concern this-is-a-test

[apiraino]

Ok, no user-visible effects. But the logic in the triagebot to gather concerns needs the @rfcbot invocation, just as I had implemented it since the beginning

[ehuss]

Ok, I see, thanks!